fix(p3): async file lock for event loop blocking (Issue #763)

[jlin53882]

Summary

Issue #763 P3: Event Loop Blocking in runWithFileLock()

Convert 5 synchronous I/O calls to async versions to prevent event loop blocking:

Before (Sync)	After (Async)
existsSync	pathExists() [access]
mkdirSync	await mkdir()
writeFileSync	await writeFile()
statSync	await stat()
unlinkSync	await unlink()

Changes

• Add node:fs/promises imports (access, mkdir, stat, unlink, writeFile)
• Add pathExists() static helper (async equivalent of existsSync)
• Convert init block to async (mkdir + writeFile)
• Convert stale check to async (stat + unlink)
• Add unit tests (5 tests pass)

Tests

node --test test/p3-async-file-lock.test.mjs
# 5/5 tests passed

Fixes

• P3 (Issue [perf] MMR O(n³)→O(n²) + Event Loop Blocking 防治（P2+P3 合併追蹤） #763): async file lock prevents event loop blocking in high-concurrency scenarios
• Previous PR fix(p3): async file lock for event loop blocking (Issue #763) #772 mixed retriever changes; this is a clean P3-only fix

Checklist

• Security: No vulnerabilities
• Performance: Async I/O prevents blocking
• Correctness: Error handling preserved
• Testing: 5 unit tests pass
• CI manifest updated