perf(scan): rewrite walker hot path with sync I/O + manual string ops

[BYK]

Summary

Walker was 411ms p50 for 10k files on the synthetic/large fixture — roughly 10× slower than a naive sync walk (50ms). Follow-up to PR #791 and #804.

Five targeted optimizations in the hot loop, all micro-benchmarked before landing:

Change	Measured win	End-to-end win
readdirSync instead of readdir	105→45ms (−60ms)	dominant
statSync instead of Bun.file().size	−10ms per 10k files
String concat for paths	path.join 7ms → concat 0.7ms (10×)
abs.slice(cwdPrefixLen) for relPath	path.relative 9ms → slice 0.8ms (11×)
Manual lastIndexOf-based extname	9ms → 7ms (25%)

Perf results (synthetic/large, 10k files, p50)

Op	Before	After	Δ
scan.walk	411ms	231ms	−44%
scan.walk.noExt	572ms	448ms	−22%
scan.walk.dsnParity	228ms	138ms	−39%
scanCodeForDsns	323ms	304ms	−6%
detectAllDsns.cold	327ms	308ms	−6%
detectAllDsns.warm	27.9ms	27.0ms	—
scan.grepFiles	322ms	316ms	noise
scanCodeForFirstDsn	2.23ms	2.05ms	—

Walker ops are 22-44% faster. Downstream ops (grep, DSN scanner) benefit less because their time is dominated by content scanning, not walking — but still show consistent ~6% improvements.

Why readdirSync is safe here

The sync vs async tradeoff usually favors async because blocking the event loop is bad in general. But measured per-call cost matters:

Metric	readdirSync on 3635 dirs
p50	11µs
p95	24µs
p99	35µs
max	65µs

65µs max block is trivial — setTimeout(0) latency in Node is ~4ms. Blocking for 65µs never causes noticeable event-loop pauses. And we pay ~60µs of microtask overhead for each async readdir, which wipes out any theoretical fairness benefit. Net: 2-3× faster per-dir on walks with many small directories, which is every realistic CLI workload.

If this ever matters for a weird embedded use case, the optimization is trivially reversible — the walker's public API is unchanged.

mtime parity fix

Initial implementation regressed detectAllDsns.warm from 28ms → 304ms because statSync().mtimeMs is a float (e.g. 1776790602458.1033) while Bun.file().lastModified is already an integer. The DSN cache validator compares floored sourceMtimes, so un-floored floats caused cache misses on every warm call. Fixed by flooring explicitly in tryYieldFile — matches the same treatment already applied to onDirectoryVisit's dirMtimes.

Walker v2 design notes

• Sync readdir + async yield. Readdir blocks briefly (~65µs max), but the surrounding async-generator interface is preserved. Library consumers that insert async work between iterations still get cooperative scheduling.
• Path ops inlined. path.join / path.relative / path.extname + toLowerCase are all replaced with manual string ops in the hot loop. On Windows, normalizePath is still applied via replaceAll(NATIVE_SEP, "/") — the POSIX fast path uses a cached POSIX_NATIVE module constant.
• WalkContext.cwdPrefixLen precomputed once per walk, used to slice relative paths from absolute paths.
• Cached module constants NATIVE_SEP and POSIX_NATIVE avoid per-call path.sep property lookups in the hot loop.

What this PR does NOT change

• WalkEntry shape: preserved identically (absolutePath, relativePath, size, mtime, isBinary, depth).
• WalkOptions contract: no new options, no semantics changes.
• Symlink handling, followSymlinks, cycle detection via visitedInodes: unchanged.
• IgnoreStack + gitignore semantics: unchanged.
• onDirectoryVisit, recordMtimes, abortSignal: unchanged.

Test plan

• bunx tsc --noEmit — clean
• bun run lint — clean (1 pre-existing warning in src/lib/formatters/markdown.ts unrelated)
• bun test --timeout 15000 test/lib test/commands test/types — 5640 pass, 0 fail
• bun test test/isolated — 138 pass
• bun test test/lib/scan/walker.test.ts — 34 pass (incl. property tests covering hidden files, symlinks, maxDepth, gitignore interaction)
• bun test test/lib/dsn/code-scanner.test.ts — 52 pass (incl. dirMtimes / sourceMtimes cache validation)
• DSN count correctness verified end-to-end: 4 DSNs found on fixture (matches pre-change count)
• bun run bench --size large --runs 5 --warmup 2 — results in table above

🤖 Generated with Claude Code

Co-authored-by: Claude Opus 4.7 (1M context) noreply@anthropic.com

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Codecov Results 📊

✅ 138 passed | Total: 138 | Pass Rate: 100% | Execution Time: 0ms

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ No test changes detected

All tests are passing successfully.

✅ Patch coverage is 85.71%. Project has 1772 uncovered lines.
❌ Project coverage is 95.62%. Comparing base (base) to head (head).

Files with missing lines (1)

File	Patch %	Lines
src/lib/scan/walker.ts	85.71%	⚠️ 5 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
- Coverage    95.63%    95.62%    -0.01%
==========================================
  Files          281       281         —
  Lines        40442     40463       +21
  Branches         0         0         —
==========================================
+ Hits         38676     38691       +15
- Misses        1766      1772        +6
- Partials         0         0         —

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