Fix Markdown heading fragment resolution

[Widthdom]

Summary

• Index Markdown headings and explicit HTML anchors as path-scoped definitions.
• Resolve local and cross-document fragment links without same-name dependency fanout.
• Derive heading slugs from rendered inline text while preserving exact explicit anchor identity.
• Repair version-4 reference identity state and refresh exact Markdown anchors when only the target file changes.
• Preserve the NuGet lock graph contract and candidate behavior integrated from the latest main.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• Release focused regression tests: net8.0 2,714 passed; net9.0 2,689 passed, 25 skipped
• Full Release test suite:
  • net8.0: 10,599 passed, 7 skipped, 0 failed
  • net9.0: 10,126 passed, 419 skipped, 0 failed
• dotnet run --project tools/CodeIndex.Changelog -- check
• cdidx status --check --json reports a fresh index matching the workspace
• CLI smoke: definition error-codes --lang markdown --json returns the USER_GUIDE.md heading declaration
• Codex adversarial review: two rounds completed; all actionable findings from both rounds were addressed and covered by regression tests

Documentation and changelog

• Updated the English and Japanese Markdown behavior sections in USER_GUIDE.md and DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/4846.fixed.md.

Follow-ups

• None.

Fixes #4846