chore: Added diagnostics plan.

DIAGNOSTICS.md

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+# Diagnostics
+
+Thor has two diagnostic tiers. This document explains why, and when to use each.
+
+## Tier 1: Rust-style rich diagnostics (mustache engine)
+
+`mustache/diagnostic.odin` implements multi-line source context, caret underlines, ANSI colors (gated on TTY detection), and Levenshtein suggestions. Used exclusively by the mustache engine for template errors:
+
+- Unknown keys in `{{k}}`, `{{{k}}}`, `{{#k}}`, `{{^k}}`
+- Missing partials (`{{> name}}`)
+- Missing parent templates (`{{<name}}`)
+- Unmatched block overrides (`{{$name}}`)
+- Parse-time syntax errors
+
+These benefit from rich diagnostics because **exact source location matters** — templates have complex syntax, and the user often doesn't know *where* the problem is. The diagnostic system operates on `Template.source` with byte offsets, producing output like:
+
+```
+error: unknown key 'titel' in {{page.titel}}
+  --> layouts/page.html:12:22
+   |
+12 |   <h1>{{page.titel}}</h1>
+   |                  ^^^^^
+   |
+   = hint: did you mean 'title'?
+```
+
+## Tier 2: Simple log warnings (content and runtime)
+
+Everything outside the mustache engine uses `log.warnf` — flat one-line messages via `core:log`:
+
+- Missing frontmatter dates (fallback to file mtime)
+- Duplicate menu weights
+- Non-numeric weight values in frontmatter
+- Tree-sitter highlight errors
+- Menu system issues (mixing config/frontmatter menus, etc.)
+
+These are simple, actionable, and cross-file. The problem isn't *location* — it's that two files disagree, or a value is missing. A caret pointing at one file doesn't help; the message already communicates what to fix:
+
+```
+[WARN] --- [menus.odin:290:warn_duplicate_weights()] menus('main'):'Ideas' and 'Stuff' share the same weight (11).
+```
+
+## Why not use rich diagnostics everywhere?
+
+Rust's diagnostic model is built for a single compilation unit with full AST/IR data. Three obstacles prevent reusing it for content warnings:
+
+1. **Source tracking**: The mustache diagnostic system operates on `Template.source` (byte offsets into template strings). Content warnings come from frontmatter in markdown files — different source, different parser, no position tracking. Reusing the system would require building a parallel position-tracking infrastructure for frontmatter.
+
+2. **Cross-file context**: Rust diagnostics point at one location. Weight duplicates are a relationship between two files. Rich diagnostics would need to show *both* file locations, which is more infrastructure for marginal value.
+
+3. **Diminishing returns**: Rust diagnostics shine for syntax/type errors where the user doesn't understand the failure. Content warnings are already self-explanatory — "these two pages share weight 11" doesn't need a caret to be actionable.
+
+## When to upgrade a Tier 2 warning to Tier 1
+
+If a warning's usefulness would significantly improve from showing exact source location (e.g., a frontmatter syntax error where the user needs to see *which line* is malformed), consider extending the diagnostic system to frontmatter. This would require:
+
+1. Position tracking in `frontmatter.odin` (store byte offsets for each parsed field)
+2. A `format_frontmatter_error` proc modeled on `format_render_error`
+3. File path propagation through the page loading pipeline
+
+This is not currently planned — see `TODOS.md`.