Stabilize macOS net8 subproject watch reconciliation

[Widthdom]

Summary

• Keep macOS .NET 8 subproject project-tree delivery on FileSystemWatcher/FSEvents while polling only the exact ancestor .gitignore and .cdidxignore paths.
• Reserve full-project polling for watcher recovery attempts and preserve the .NET 9 and non-macOS backend-selection behavior.
• Add deterministic regression coverage, bilingual user/developer/testing documentation, and a CHANGELOG fragment.

Root cause

On macOS under .NET 8, the FSEvents path can silently miss changes to ancestor ignore files when watching a subproject. Extending the reconciliation wait to 60 seconds did not recover, confirming an event-delivery miss rather than slow synchronization. The fix supplements normal project-tree events with a bounded exact-path poll of ancestor ignore files, avoiding recursive idle scans of the subproject.

Validation

• Issue Stabilize net8 subproject watcher reconciliation for ancestor .gitignore changes #4955 related tests: .NET 8: 10 passed; .NET 9: 10 passed
• Watcher test class: .NET 8: 60 passed; .NET 9: 60 passed
• Repeated real .NET 8 ancestor-ignore integration runs: passed
• Full Release suite: .NET 8: 10,879 passed, 7 skipped, 0 failed; .NET 9: 10,405 passed, 420 skipped, 0 failed
• Release build: 0 warnings, 0 errors
• Post-commit Debug build: 0 warnings, 0 errors
• CHANGELOG fragment validation: 60 fragments valid
• Local index freshness/workspace check: matched, no failed checks
• Codex adversarial review: addressed the recursive-polling performance finding and the duplicated bilingual documentation finding; no unresolved findings remain

Documentation / changelog

• USER_GUIDE.md
• DEVELOPER_GUIDE.md
• TESTING_GUIDE.md
• changelog.d/unreleased/4955.fixed.md

Follow-up candidates

None.

Fixes #4955