fix: prevent file corruption when multiple IDE windows patch concurrently

[BenKalsky]

Problem

Each open IDE window runs its own extension host, and they all patch the same Claude Code files on disk (webview/index.js, webview/index.css, extension.js). The injection path does an unsynchronized read-modify-write (copyFile backup → readFile → writeFile) with no lock between windows/processes.

With more than one window open, these cycles interleave and tear the file. In a real report on Antigravity IDE, webview/index.js was truncated from 4.8 MB → ~1 MB right after a Claude Code reinstall (which re-triggered patching in every window). The truncated webview bundle fails to load → the Claude panel renders completely blank and sessions appear gone. Restoring from the .bak brought it back, confirming the on-disk file was the corrupted artifact.

This is separate from #11 (which fixes which directory is searched); this fixes how the files are written.

Fix

src/injector.ts:

• Serialize every mutating op behind a per-extension-directory lock file (withFileLock), so only one window patches at a time. Stale locks (crashed window) are auto-broken after 30s, and it falls through after a bounded wait rather than ever hanging the extension.
• Atomic writes (atomicWrite): write to a unique temp file then rename() into place, so a reader/racing window never observes a half-written file.
• Corruption guard: injection only adds content, so a result smaller than the pristine backup is rejected instead of written — truncation can never be persisted.
• Public functions (addRtl, addRtlAlways, addRtlAuto, fixBidi, removeRtl) are now thin lock-serialized wrappers over lock-free *Impl functions, so fixBidi composes them without re-entrant deadlock.

Test

Adds test/concurrency.test.cjs (run with npm test): patches one extension from 8 simulated windows concurrently and asserts no truncation, no stacked injections, pristine backups, and clean removal. The pre-fix code truncates/hangs under the same race; the fixed code passes.

PASS — 8 concurrent windows, no corruption
  index.js  orig=5000044 inject=5004129 remove=5000044
  index.css orig=29 inject=4929
  JS markers after race: 1 (no stacking)

Verified: tsc --noEmit clean, npm run build clean, npm test passes. Bumped to v0.4.2.

🤖 Generated with Claude Code