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+# Asset Pipeline Plan
+
+Design for letting thor users bring their own CSS toolchain (Sass, PostCSS,
+Lightning CSS, Tailwind, …).
+
+## Thesis
+
+Let thor users bring their own CSS toolchain by **orchestrating the tool they
+already installed** — never bundling or reimplementing one. This is expressed
+through thor's existing mustache pipe grammar with two new ops: `tool` and
+`fingerprint`.
+
+The earlier research design (content-addressed registry + deferred token
+resolution + a separate post-render path for "whole-output scanners") is
+**discarded**. It was overbuilt. Every tool — Tailwind included — scans
+*source*, which exists at render time, so every transform runs **eagerly,
+synchronously, and memoized**. No placeholder tokens, no post-render
+string-replace pass, no second integration surface.
+
+## The two ops
+
+```html
+<link href="{{ "css/main.scss" | tool sass | fingerprint | rel_url }}">
+```
+
+```json5
+// thor.json
+{
+  "tools": {
+    "sass":         "sass $in $out",
+    "postcss":      "postcss $in -o $out",
+    "lightningcss": "lightningcss --minify $in -o $out",
+    "tailwind":     "tailwindcss -i $in -o $out"
+  }
+}
+```
+
+## Core model
+
+The value flowing through the pipe is **bytes held by thor**, with a logical
+name attached (`css/main.css`). Each stage transforms the bytes; the **final
+stage publishes** one file to the output dir and returns its URL.
+
+- A literal (`"css/main.scss"`) or context key names the **source** file on disk.
+- `| tool <name>` runs a subprocess, replacing the carried bytes with its output.
+- `| fingerprint` writes the content-addressed file and returns its hashed URL.
+- The chain composes with the existing `rel_url` for base_url subpaths.
+
+**Memoization:** results are cached per build, keyed by (op-chain, source). The
+first page to reference an asset runs the tool; every other page hits the cache.
+This is what stops N pages from shelling out N times.
+
+## `| tool <name>`
+
+Looks up `<name>` in the `thor.json` `tools` table and runs its command
+template. `$in` and `$out` are **optional per-tool I/O adapters**:
+
+| Template has | Behavior |
+|---|---|
+| `$in` | thor writes current bytes to a scratch path, substitutes it |
+| no `$in` | thor pipes current bytes to the tool's **stdin** |
+| `$out` | thor substitutes a scratch path, reads it back after |
+| no `$out` | thor captures the tool's **stdout** |
+
+Defaulting to stdin/stdout avoids scratch-file overhead when the tool supports
+it. Tools that can't stream (Lightning CSS wants a path; Tailwind v3 needs an
+input file) just get `$in`/`$out` in their template. Either way the bytes are
+thor's, so chaining works regardless of each tool's preference:
+`| tool sass | tool postcss` composes even if sass streams and postcss wants
+files.
+
+- **`$out` is always thor-controlled scratch** — the tool never picks the
+  published filename.
+- **Extension derivation:** thor owns the output name; `main.scss` through
+  `tool sass` becomes `main.css`.
+- **Errors:** thor gates on exit code and surfaces the tool's stderr through one
+  code path. If the tool is missing from PATH, fail with its stderr — never
+  bundle, never assume a runtime.
+- **A bare `tool` (no `fingerprint`) still publishes** — to the logical unhashed
+  path (`output/css/main.css`), returning `/css/main.css`.
+
+## `| fingerprint`
+
+Content-addresses whatever bytes it currently holds. It has **two outputs**:
+
+- **Side effect:** writes `css/main.<hash>.css` to the output dir.
+- **Return value:** the hashed URL string `/css/main.<hash>.css` (feeds
+  `rel_url`).
+
+Properties:
+
+- **Works standalone**, no `tool` needed: `{{ "css/main.css" | fingerprint }}`
+  is plain cache-busting of a hand-written file — probably the most common use.
+- **Position decides what's hashed** — it hashes current bytes, so it belongs
+  **last** in the chain. No terminal-only special-casing; the rule is just "put
+  it last."
+- **Hash format:** `name.<hash>.ext` (CDN-friendly), 8 hex chars of SHA-256.
+
+## Tailwind is not special
+
+It scans your **source** (`layouts/`, `content/`, partials) — which exists at
+render time — so it runs synchronously like sass. Its `thor.json` entry just
+needs content globs pointing at the source tree (or v4 auto-detection). The
+**one** asymmetry is watch invalidation: sass re-runs when `main.scss` changes;
+Tailwind must re-run when *any* scanned source changes. That's a cache-key
+breadth difference, not an architectural one. The cost of scanning source vs.
+output is losing dynamically-assembled class tokens (`col-{{n}}`) — a Tailwind
+anti-pattern whose sanctioned fix is a safelist, not warping the build.
+
+## Interaction: `copy_assets_dir`
+
+Today thor copies everything under `assets/` to the output dir. Any source
+consumed by a pipe must be **excluded from that plain copy** — otherwise you'd
+ship the raw `main.scss` (or a dead unhashed `main.css` alongside the hashed
+one). The pipe owns the write for assets it touches.
+
+## Implementation steps (separable)
+
+1. **`| fingerprint`** — no subprocess, no config. Hash + write + return URL,
+   plus the `copy_assets_dir` exclusion. Smallest blast radius, independently
+   useful, ships first.
+2. **`| tool <name>`** — `thor.json` tools table, the `$in`/`$out`/stdin/stdout
+   adapter, subprocess + exit-code/stderr handling, extension derivation,
+   memoization. Composes with step 1.
+
+## Open questions for implementation
+
+- **Watch invalidation:** simplest correct v1 recomputes the memo each build
+  (cheap for the common single-asset case). A cross-build skip keyed on source
+  content/mtime is a later optimization — and Tailwind's broad content-glob key
+  is where it matters most.
+- **Source-map sidecars:** sass/lightning emit `.map` files next to `$out`;
+  disable in the template or ignore the sidecar. Minor.
+- **Error policy default:** hard-fail the build vs. warn-and-skip on tool
+  failure/absence. Lean fail; possibly configurable later.
diff --git a/ASSET_PIPELINE_RESEARCH.md b/ASSET_PIPELINE_RESEARCH.md
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-# Asset Pipeline Research: Bring-Your-Own CSS Preprocessor
-
-Research and design notes for letting thor users bring their own CSS toolchain
-(Sass, PostCSS/Tailwind, Less, Lightning CSS, …) instead of thor embedding or
-tracking any of them.
-
-## Problem statement
-
-The CSS pre-processor landscape is large and moving (Tailwind, PostCSS, Less,
-Sass, Lightning CSS, …). thor is a ~1.9 MB self-contained binary that tracks its
-own size (see `AUDITING_BINARY_SIZE.md`); bundling any one preprocessor — let
-alone keeping up with all of them — is a non-starter on both size and
-maintenance grounds. So "bring your own" has to mean **thor orchestrates the
-tool the user already installed**, never reimplements it.
-
-Today `copy_assets_dir` (`assets.odin`) copies everything under `assets/` to the
-output dir and optionally minifies `.css` via the tree-sitter pass in
-`minify.odin`. There is no preprocessing seam.
-
-## What we are NOT doing, and why
-
-- **Bundle a specific preprocessor** — rejected: binary-size-hostile, and picks
-  winners in a space we said we can't keep up with.
-- **Hugo Pipes-style template resource pipeline** (`resources.Get | css.Sass |
-  minify | fingerprint`) — powerful, but needs a resource abstraction and a
-  non-logic-less template language to host the chained syntax. thor's engine is
-  logic-less mustache; wrong fit.
-- **Curated shell-out for named tools only** (what Hugo actually ships — only
-  `css.Sass` and `css.PostCSS` shell out) — this just re-picks winners.
-
-The chosen direction reuses thor's **existing mustache pipe grammar** (the same
-mechanism as the `rel_url` pipe) to express asset transforms declaratively,
-where the asset is referenced.
-
-## Correcting the record on Hugo
-
-An earlier draft attributed a generic "pre/post-build shell hook" model to Hugo.
-That is wrong. Hugo has **no** generic build hooks. Its CSS handling is Hugo
-Pipes — a *template-invoked resource pipeline* of fixed, named functions
-(`css.Sass`, `css.PostCSS`, `minify`, `fingerprint`, `resources.Concat`, …).
-Only two of those shell out: `css.PostCSS` (to a local `postcss-cli` under Node)
-and `css.Sass` (embedded LibSass, or a `dart-sass` binary). Generic
-"run any command pre/post build" is closer to npm-scripts / Makefile glue, or
-Eleventy's `before`/`after` events (which run JavaScript, not shell commands).
-No mainstream SSG markets a generic command hook as its asset story.
-
----
-
-## Tool CLI research
-
-Five representative tools were researched for their exact shell-out contract.
-The questions that determine thor's interface are the same for each: stdin/stdout
-vs. file-path args, whether a config file is mandatory, whether it needs to scan
-rendered HTML, watch support, minify overlap, and runtime dependencies.
-
-### Mechanical matrix
-
-| Tool | stdin→stdout | Config file | Self-contained (no Node) | Self-minifies | Errors |
-|---|---|---|---|---|---|
-| **Dart Sass** | yes (`sass -`) | none | yes — standalone binary | `--style=compressed` | stderr, exit 65 |
-| **PostCSS** (`postcss-cli`) | yes (`< in > out`) | optional (`--use`) | **no** — needs Node + node_modules | via cssnano plugin | stderr, exit 1 |
-| **lessc** | yes (`-` / positional) | none | **no** — needs Node | weak/deprecated (`-x`) | stderr, nonzero |
-| **Lightning CSS** | file-in, stdout-out | none (flags) | yes — Rust binary (npm-distributed) | `--minify` (its core purpose) | stderr, nonzero |
-| **Tailwind v4** | yes (`-i - -o -`) | none (CSS-first) | yes — standalone binary (Bun) | `--minify` | stderr, nonzero |
-| **Tailwind v3** | no (needs `-i` file) | `tailwind.config.js` | yes — standalone (pkg/Node embed) | `--minify` | stderr, nonzero |
-
-Two properties are uniform and load-bearing:
-
-- **Every tool writes errors to stderr and returns nonzero on failure.** thor can
-  gate on exit code and surface stderr through a single code path for all tools.
-- **Every tool can stream to stdout.** Differences (Lightning CSS wants a path
-  not stdin; lessc's output is positional; Tailwind v3 needs an input file) are
-  all absorbable by a per-tool command template with placeholders.
-
-### Per-tool detail
-
-**Dart Sass** — `sass input.scss output.css`, or `sass -` for stdin, prints to
-stdout when no output arg. No config file exists or is supported; configured by
-CLI flags + `@use`/`@import` in source. Load paths via `--load-path`/`-I`.
-`--style=compressed` minifies (overlaps thor's minifier). Source maps on by
-default (`--no-source-map` to disable). Exit 64 = usage error, 65 = compile
-error. **Distribution:** standalone release bundles the Dart VM + snapshot, no
-Node; the npm `sass` package is dart2js and needs Node. Prefer the standalone.
-
-**PostCSS** (`postcss-cli`, binary `postcss`) — positional input files or stdin;
-stdout is the default when no `-o`/`--dir`/`--replace`. Config (`postcss.config.js`
-/ `.postcssrc`, cosmiconfig, walks up) is **not required** — plugins can be given
-with `-u/--use` (e.g. `postcss in.css -u autoprefixer -u cssnano -o out.css`).
-Zero plugins = near pass-through. **postcss is only a plugin host** — autoprefixer,
-tailwind, cssnano are separate npm packages resolved from `node_modules`.
-`-w/--watch` tracks the full `@import` dependency graph via PostCSS `dependency`
-messages. Exit 1 on error, errors to stderr, stdout stays clean for piping.
-**Hard Node + node_modules dependency.** Invoke via `npx postcss` to resolve
-local `.bin`.
-
-**lessc** — `lessc [opts] <source> [destination]`; `-` reads stdin; stdout when
-destination omitted (**no `-o` flag** — output is positional). No config file;
-`.less` source + flags. `--include-path` for import search. `-x`/`--compress` is
-deprecated/weak — let thor's minifier compress. **No CLI watch mode** (watch only
-exists in the browser runtime). Errors to stderr, nonzero on compile error.
-**Hard Node dependency** (`bin/lessc` is a Node script).
-
-**Lightning CSS** (`lightningcss`) — input file positional, `-o`/`--output` or
-stdout when omitted. No config, flag-driven. `--minify` is the tool's core
-purpose (**directly overlaps thor's minifier** — pick one). `--bundle` inlines
-`@import`s; `--targets "<browserslist>"` downlevels modern CSS. No built-in watch.
-Errors to stderr, nonzero. **Runtime: self-contained Rust binary, no Node to
-run** — but distribution is npm-only (`lightningcss-cli` pulls a platform package
-like `lightningcss-cli-linux-x64-gnu`); no official GitHub/Homebrew/Winget
-binaries. Can extract the binary from the npm registry tarball and run it
-standalone.
-
-**Tailwind CSS standalone CLI** (v3 and v4) — both ship a self-contained
-per-platform binary (`./tailwindcss`), no Node required (v3 via Vercel pkg, v4
-via Bun). `-i`/`--input`, `-o`/`--output` (defaults to stdout in v4). v4 reads
-stdin with `-i -`; v3 effectively needs an input file. Input is a CSS entry file:
-v3 uses `@tailwind base/components/utilities;`, v4 uses `@import "tailwindcss";`.
-**Config:** v3 uses `tailwind.config.js` (auto-discovered, `-c` to override) and
-in practice needs it (or `--content`) to know what to scan; v4 is CSS-first and
-genuinely zero-config. `--minify` in both. Errors to stderr, nonzero (codes not
-formally enumerated — check `exit != 0`).
-
-**Content scanning (the critical Tailwind property):** Tailwind scans
-source/template files **as plain text** for complete, static class-name tokens.
-It does not parse code and does not strictly require rendered HTML — it scans
-whatever files you point it at. But for an SSG the safe strategy is to scan the
-**generated output** after render, so classes that only appear post-render aren't
-missed. v3: `content: ["./public/**/*.html"]` or `--content`. v4: automatic
-detection, but it **ignores `.gitignore`d paths** — so a git-ignored `public/`
-must be added explicitly with `@source "../public"`.
-
----
-
-## Design conclusions
-
-### The one split that matters: two tool shapes
-
-The research draws a hard line between two categories that need **different
-integration surfaces**:
-
-1. **Per-asset transformers** — Sass, Less, Lightning CSS, PostCSS-as-compiler.
-   Input = one source file → output = one CSS file. These fit a mustache pipe:
-   `{{ "css/main.scss" | tool sass }}`. Each tool's CLI quirk (stdin vs. path,
-   positional output, etc.) is absorbed by a per-tool command template.
-
-2. **Whole-output scanners** — Tailwind, alone. Its "input" is not one asset; it
-   is an entry CSS file **plus every rendered HTML file it scans**. At the moment
-   a per-page pipe would fire, the full output does not exist yet. Tailwind must
-   run **once, after all HTML is written, pointed at `output_dir`**. That is a
-   post-render step, categorically not a pipe.
-
-### The chicken-and-egg, and the fix
-
-A `<link>` wants a **fingerprinted (cache-busted) URL**, but for Tailwind the CSS
-isn't produced until **after** render — so at render time the template cannot
-know the hash. Hugo dodges this with a lazy multi-pass resource graph; thor is
-single-pass.
-
-**Resolution — emit a placeholder at render, resolve at the end.** A pipe like
-`{{ "css/main.scss" | tool sass | fingerprint }}` writes a **stable token** (e.g.
-`⟦asset:css/main.scss⟧`) into the HTML. After render, thor runs the tools,
-populates a **content-addressed asset registry** (logical name → final hashed
-URL), then does one cheap string-replace pass over the output swapping tokens →
-real URLs.
-
-This single mechanism dissolves every constraint encountered:
-
-- **Cache-busting:** the hash is known at resolve time, so the token becomes
-  `/css/main.a1b2c3.css` uniformly, even for tools that rename output themselves.
-- **Hash-after-minify:** resolution is the last step, so the hash is always of the
-  exact final served bytes.
-- **Same asset → same hash across N pages:** the registry is computed once; every
-  token resolves identically. (This is also the memoization that keeps a
-  per-page pipe from shelling out N times for the same file.)
-- **Tailwind:** its post-render output lands in the registry like any other asset;
-  tokens that referenced it resolve correctly despite the CSS not existing at
-  render time.
-
-### Cache-busting: two regimes
-
-- **Regime A — thor owns the hash (recommended, and what every researched
-  compiler needs).** Tool → stdout → thor minifies → thor hashes the final bytes →
-  writes `main.<hash>.css` → registers the URL. None of Sass/PostCSS/Less/
-  Lightning CSS/Tailwind fingerprint on their own; fingerprinting is a
-  bundler/plugin behavior. thor stays in full control of the name because it
-  writes the file. Mirrors Hugo's `| fingerprint` → `.RelPermalink`.
-- **Regime B — the tool owns the hash** (bundlers: esbuild/vite/parcel, or
-  PostCSS + `postcss-hash`). The tool renames output to a name thor can't predict,
-  and stdout capture doesn't reveal it. The standard answer these tools already
-  provide is a **manifest** (`manifest.json` mapping `"main.css"` →
-  `"main.a1b2c3.css"`). Supporting this class means thor ingests that manifest
-  into the same registry rather than scanning for files.
-
-Both regimes converge on the same abstraction: **a build-scoped, content-addressed
-asset registry** (logical name → final emitted URL), populated by thor's own
-transform+fingerprint or by an external tool's manifest, and resolved through the
-placeholder pass.
-
-## Proposed interface shape
-
-- **`thor.json` `tools` table** — one command template per tool. Keeps shell
-  commands out of templates (a single trust surface), and absorbs each CLI's
-  quirks. `$in` = source path; stdout is captured:
-
-  ```json
-  "tools": {
-    "sass":         "sass --no-source-map $in",
-    "postcss":      "postcss -u autoprefixer $in",
-    "lightningcss": "lightningcss --minify $in",
-    "tailwind":     { "run": "tailwindcss -i $in -o -", "scan": "output" }
-  }
-  ```
-
-  The `scan: output` marker flips a tool from per-asset (pipe) to post-render
-  (whole-output) mode.
-
-- **Pipe ops** — `tool <name>` (transform → asset handle), `minify`,
-  `fingerprint` — all resolving through the registry, chainable with the existing
-  `rel_url`:
-
-  ```
-  <link href="{{ "css/main.scss" | tool sass | minify | fingerprint | rel_url }}">
-  ```
-
-  A lookup-direction op resolves a logical name to its final URL:
-  `{{ "css/main.css" | asset }}` → `/css/main.a1b2c3.css`.
-
-- **Final resolve pass** over `output_dir` replacing asset tokens with registry
-  URLs. This is also where post-render scanners (Tailwind) have already run.
-
-### Fit with the existing mustache pipe grammar
-
-`{{ "css/main.css" | tool postcss }}` already parses under `mustache/pipes.odin`:
-the key is a quoted literal (`tokenize_fields` keeps the quotes so a literal is
-distinguishable from a context key), `tool` is the op, `postcss` is one arg
-(under `MAX_PIPE_ARGS = 2`). Adding an op is the documented one-liner path — a
-variant on `Pipe_Op` plus a `case` in `apply_filter`. The only architectural
-departure from existing ops (all pure value→value string transforms) is that
-`tool`/`fingerprint` have side effects (subprocess, file write) and must be
-backed by the registry — which is why the placeholder + deferred-resolve model
-matters: it keeps render itself side-effect-light and pushes the real work to a
-defined post-render phase.
-
-## Cross-cutting decisions
-
-1. **Node dependency is not thor's problem.** PostCSS and lessc require Node +
-   node_modules; Sass/Lightning CSS/Tailwind ship self-contained binaries. thor's
-   job is to exec whatever is on PATH and **fail gracefully with the tool's
-   stderr** if it's absent — never bundle, never assume a runtime. The Nix flake
-   makes pinning the toolchain pleasant for users who want reproducibility.
-2. **Minification must be explicit, never doubled.** Every one of these tools can
-   self-minify, and thor has its own minifier. Make `minify` an explicit pipe
-   step and do **not** auto-apply thor's global minify to tool-produced assets —
-   otherwise `| tool lightningcss` (already minified) gets re-minified. The pipe
-   chain is the single source of truth for whether/when an asset is minified.
-3. **thor drives the watch; ignore the tools' own `--watch`.** thor already polls
-   (5 s). Invoke each tool once per build as a pipeline step. Sass/PostCSS/
-   Tailwind have watch modes, but running them would put a second watcher in
-   competition with thor's loop — avoid it.
-
-## Headline
-
-The feature is not "a `tool` pipe." It is a **content-addressed asset registry
-with deferred token resolution**, exposed through two pipe ops (`tool`,
-`fingerprint`/`asset`) plus one post-render hook (for whole-output scanners like
-Tailwind). That is the minimum that handles both tool shapes *and* cache-busting
-without introducing a multi-pass render engine.
-
-## Open questions for the design phase
-
-- Placeholder token format and collision-safety (must survive HTML minification
-  untouched — likely treat like the `pre`/`code` preserve ranges in `minify.odin`).
-- Registry lifetime and keying (content hash vs. path+mtime) and how it threads
-  through the watch loop for incremental rebuilds.
-- Whether `fingerprint` is terminal-only, or the resolve pass always hashes
-  post-minify regardless of chain position.
-- Manifest ingestion format for Regime B (adopt Vite/webpack `manifest.json`
-  shape?).
-- Error policy: hard-fail the build vs. warn-and-skip when a tool exits nonzero
-  or is missing from PATH (probably configurable, defaulting to fail).
-
-## Sources
-
-- Tailwind CLI (v4): https://tailwindcss.com/docs/installation/tailwind-cli ·
-  content detection: https://tailwindcss.com/docs/detecting-classes-in-source-files ·
-  standalone: https://tailwindcss.com/blog/standalone-cli ·
-  v3: https://v3.tailwindcss.com/docs/installation ·
-  https://v3.tailwindcss.com/docs/content-configuration
-- PostCSS CLI: https://github.com/postcss/postcss-cli (README + `index.js` +
-  `lib/args.js`)
-- Dart Sass CLI: https://sass-lang.com/documentation/cli/dart-sass/ ·
-  https://github.com/sass/dart-sass
-- Less: https://lesscss.org/usage/ ·
-  https://github.com/less/less-docs/blob/master/content/usage/command-line-usage.md
-- Lightning CSS: https://lightningcss.dev/docs.html ·
-  https://github.com/parcel-bundler/lightningcss ·
-  https://www.npmjs.com/package/lightningcss-cli ·
-  distribution discussion: https://github.com/parcel-bundler/lightningcss/discussions/732