Fix HTML class reference extraction

[Widthdom]

Summary

• Split HTML class and className attribute values into individual reference symbols.
• Added coverage for double spaces, Tailwind-like variants, and empty class attributes.
• Merged latest main and resolved the HTML markup extractor conflict.
• Fixed the newly surfaced Rust trait associated type default CI failure; see Rust associated type defaults are not emitted for trait symbols #2603.

Fixes #2103
Refs #2603

Documentation and changelog

• Added changelog fragment: changelog.d/unreleased/2103.fixed.md
• Added changelog fragment: changelog.d/unreleased/2603.fixed.md
• No README/user guide update was needed because these are indexing correctness fixes, not new commands or options.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~SymbolExtractorTests.Extract_HtmlClassAttributes_IndexesIndividualClassReferences
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~SymbolExtractorTests.Extract_RustTraitAssociatedTypeDefaults_RecordsPropertySymbols
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --filter FullyQualifiedName~SymbolExtractorTests.Extract_RustTraitAssociatedTypeDefaults_RecordsPropertySymbols -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• codex exec review --base origin/main (initial pass completed before this update; rerun after merge was stopped due node_repl timeout)