perf(findr): Each thread gets its own buffer.

PERFORMANCE_IDEAS.md

@@ -1,13 +1,22 @@
-findr is 4.5x slower than fd (case 1: 658ms vs 146ms). Opportunities:
-- Per-thread result buffers (eliminate mutex contention)
-- Arena allocator for path strings
-- Larger getdents buffer (8KB → 64KB+)
-- Buffered stdout output
+findr is ~2.3x slower than fd (case 1: 547ms vs 241ms). Opportunities:
 
-- Write while walking rather than waiting until the end?
+1. Per-thread result buffers (DONE)
+Each thread accumulates results locally, then merges once at exit. Eliminates per-result mutex contention.
 
-1. Per-thread result buffers (biggest win)
-Every result append currently takes results_mutex. With millions of files, that's millions of lock/unlock cycles. Fix: each thread accumulates results locally, then merges once when done.
+2. Batched channel (fd's approach)
+Replace global results array + merge with a buffered channel of batches. Each worker fills a local batch (~256 items), sends it to a `chan.Chan([]string)` (capacity = 2 × threads). A receiver thread drains batches and collects/prints. Provides backpressure, streaming output, and per-batch (not global) synchronization. Enables sorting like fd does (buffer first 1000 results or 100ms, then stream).
+
+3. Path allocation waste (join_path/join_path_dir)
+Every path construction spins up a strings.Builder, does fmt.sbprintf, to_string, clone, then builder_destroy — 2 heap allocs + 2 frees per path. Could be a simple memcpy into a stack buffer with a single alloc.
+
+4. Larger getdents buffer
+Currently 8KB. Increasing to 64KB+ means fewer syscalls per directory with many entries.
+
+5. Eliminate entry name cloning
+strings.clone(name) in read_dir_entries heap-allocates per dirent. Names are valid in the getdents buffer during process_dir, so the clone may be unnecessary.
+
+6. Arena allocator per thread
+Replace the default allocator for transient strings with a bump allocator — allocate in bulk, free all at once.
 2. Path allocation waste (join_path/join_path_dir)
 Every path construction spins up a strings.Builder, does fmt.sbprintf, to_string, clone, then builder_destroy — 2 heap allocs + 2 frees per path. Could be a simple memcpy into a stack buffer with a single alloc.
 3. Larger getdents buffer