Preserve YAML sequence hierarchy in structured symbols

[Widthdom]

Summary

• Preserve logical YAML sequence paths such as jobs.build.steps[14].with.path while assigning each emitted symbol to its nearest emitted mapping ancestor.
• Count scalar, empty, alias, anchor, and block-scalar sequence items without emitting synthetic item symbols; suppress block-scalar payloads and preserve sibling mappings after inline sequence mappings.
• Bump the YAML extractor contract to v3 so unchanged indexes created with the old hierarchy contract are refreshed automatically.
• Add coverage for root and nested sequences, mappings, anchors, aliases, multiline and empty items, range/order/paging, and stale-index upgrades.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
  • Passed with 0 warnings and 0 errors after the final origin/main merge.
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --no-build --no-restore -p:UseSharedCompilation=false --filter FullyQualifiedName~Issue4873
  • 3 passed.
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-build --no-restore -p:UseSharedCompilation=false --filter FullyQualifiedName~Issue4873
  • 3 passed.
• Post-merge YAML/JSON hierarchy and extractor-contract integration set
  • 14 passed on net8.0.
  • 14 passed on net9.0.
• YAML extractor and outline regression set
  • 9 passed on net8.0.
  • 9 passed on net9.0.
• Full Release suites after the review fix
  • net8.0: 10,810 passed, 7 skipped, 1 unrelated existing watcher test failed.
  • net9.0: 10,336 passed, 420 skipped, 1 unrelated existing watcher test failed.
  • The failure in both runs was RunCore_SubprojectObservesAncestorIgnoreFileChanges_Issue4592. It passed when rerun alone on net9.0; on net8.0 it also failed alone on the latest origin/main, confirming that it is not introduced by this patch.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
  • Validated 51 fragments.
• Local CodeIndex status check
  • index_matches_workspace: true
  • failed_checks: []
  • fold/graph/issues readiness: ready

Documentation and changelog

• Updated the English and Japanese YAML outline documentation in USER_GUIDE.md.
• Updated the English and Japanese regression coverage notes in TESTING_GUIDE.md.
• Added changelog.d/unreleased/4873.fixed.md.

Adversarial review

• Round 1 found that unchanged YAML files could retain stale v2 hierarchy rows; the YAML extractor contract bump and full-scan regression test address it.
• Round 2 returned no findings and rated the patch correct (confidence 0.92).

Follow-ups

• Existing Issue Use one membership and invalidation policy for scan, status, and watch #4592 covers the unrelated watcher behavior observed during the final full-suite runs; no new issue was opened.

Fixes #4873