Fix polling watch directory symlink tracking

[Widthdom]

Summary

• Follow directory symlinks in the polling watch backend when allowed by --follow-symlinks internal|all.
• Preserve the full scanner's depth-first lexical alias while bounding cycles and duplicate targets by resolved directory identity.
• Exclude internal database artifacts reached through directory aliases without canonicalizing every regular file on each poll.
• Add cross-policy regression coverage and update the English/Japanese user, developer, testing, and changelog documentation.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter FullyQualifiedName~Issue5124 (net8/net9: 10 passed each)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --no-build --filter FullyQualifiedName~IndexWatchRunnerTests (net8/net9: 88 passed each)
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false (0 warnings, 0 errors)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog/CodeIndex.Changelog.csproj -- check
• cdidx status --check and cdidx workspace status --check against the repository root index
• Full Release solution test: net9 passed 11,276 with 423 skipped; net8 passed 11,754 with 7 skipped and one unrelated fixture timeout. The isolated ReleaseWorkflow_DockerEntrypointDropsRootToRequestedUidGid rerun passed on net8 and skipped on net9.

Review

• Completed the required two-round Codex adversarial review.
• Addressed directory-alias internal-artifact filtering, full-scanner alias ordering, polling hot-path canonicalization, and stale bilingual documentation findings.

Documentation and changelog

• Updated USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md in English and Japanese.
• Added changelog.d/unreleased/5124.fixed.md.

Follow-up candidates

• None.

Fixes #5124