Bound and cancel case-sensitivity probes

[Widthdom]

Summary

• Bound existing-child case-sensitivity probes to a named 4,096-entry traversal budget while retaining only one exact-name set.
• Return an unknown result for truncated snapshots so callers use their existing isolated-write or cached root-policy fallback instead of classifying from partial data.
• Propagate available cancellation tokens through Git and file-indexing case probes, including prompt cancellation before isolated probe artifacts are created.
• Add cross-platform regression coverage for below-limit compatibility, cap look-ahead, safe fallback, and cancellation.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore -m:1 -nodeReuse:false (0 warnings, 0 errors)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --framework net8.0 --no-restore -p:UseSharedCompilation=false -m:1 -nodeReuse:false (11,821 passed, 9 skipped, 0 failed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter FullyQualifiedName~PathCompatibilityMatrixTests --no-restore -p:UseSharedCompilation=false -m:1 -nodeReuse:false (19/19 passed on net8.0 and net9.0)
• Focused FileIndexer/GitHelper cancellation and fallback tests (7/7 passed on net8.0)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• cdidx audit filesystem-traversal --path src/CodeIndex/Indexer/Scanning/CaseSensitivityProbeDirectory.cs (0 findings)
• Post-commit Debug solution build (0 warnings, 0 errors), full workspace index refresh, fold backfill, and healthy root/workspace status checks
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Updated the English and Japanese resource-boundary contracts in DEVELOPER_GUIDE.md.
• Updated the English and Japanese test contracts in TESTING_GUIDE.md.
• Added bilingual fragment changelog.d/unreleased/5160.fixed.md.

Follow-up candidates

• None.

Fixes #5160