fix: detect concurrent destination changes

[cervantesh]

Summary

• capture destination existence, file identity, byte length, and SHA-256 digest when loading filesystem edits
• revalidate immediately before atomic replacement and return stable [E_CONCURRENT_DESTINATION] recovery output without overwriting detected concurrent state
• add deterministic fixtures for same-size/restored-timestamp content changes, same-content inode replacement, deletion, and missing-to-created races
• update generated dist, README, architecture/operations guidance, and changelog

Concurrency guarantee

This is best-effort optimistic detection, not compare-and-swap. A residual race remains between final revalidation and rename; the documentation explicitly avoids a false CAS guarantee.

Validation

• npm run check
• npm run build
• npm run test:coverage (87.45% lines, 85.00% branches overall)
• npm run verify:package (28 packed files)
• npm audit --audit-level=high (0 vulnerabilities)
• git diff --check
• built distribution import smoke check

Closes #34

[chatgpt-codex-connector]

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