Prevent duplicate macOS watch startup scans

[Widthdom]

Summary

• Start the watch backend before the single required baseline scan and preserve that baseline across recoverable macOS fallback.
• Replace failed FSEvents generations, including late and ancestor-watcher failures, with an operational polling backend and schedule only one justified handoff recovery scan.
• Prune ignored/internal trees from polling, preserve terminal cancellation events, and expose backend/recovery diagnostics in JSON and human output.
• Add deterministic fake-backend coverage, guarded macOS integration coverage, bilingual documentation, and a bilingual changelog fragment.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false — passed, 0 warnings / 0 errors after the final origin/main merge.
• Watch suite in Release — 55 passed / 0 failed on both net8.0 and net9.0 after the final merge.
• Full Release suite under the preceding main snapshot completed with net8.0: 10,690 passed / 7 skipped / 1 failed and net9.0: 10,217 passed / 419 skipped / 1 failed. The net9.0 failure was the existing Issue Make extractor registration immutable and workspace-scoped #4602 timing fixture and passed in isolation on both TFMs. The net8.0 failure exposed an ancestor FSEvents error path; that path was fixed, its isolated test passed, and the complete watch suite then passed on both TFMs.
• dotnet run --project tools/CodeIndex.Changelog -- check — passed (33 fragments).
• Local cdidx status --check --json — index matches the 1,250-file workspace and final HEAD.

Adversarial review

• Codex round 1 found three actionable startup/fallback findings; all were addressed.
• Codex round 2 found four actionable late-error, polling, cancellation, and deterministic-test findings; all were addressed.
• The workflow review limit is two rounds, so post-review changes were verified with focused Release tests and final diff checks.

Documentation

• Updated both English and Japanese sections of USER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4858.fixed.md.

Fixes #4858