Fix HTML slot indexing

[Widthdom]

Summary

• Index HTML <slot name="..."> declarations as property symbols.
• Index unnamed default slots as "(default)" property symbols.
• Index slot="..." projection attributes as reference symbols.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests.Extract_Html_CapturesSlotDeclarationsAndProjectionReferences"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test

Documentation / Changelog

• Added changelog.d/unreleased/2107.fixed.md.
• No README update: this repository does not document each individual HTML symbol extraction rule in README.

Review

• Manual adversarial review of origin/main..HEAD: no blocking/actionable issues found.
• codex exec adversarial review was attempted but blocked by the current Codex usage limit.

Follow-up Candidates

• Existing issue cdidx index stalls on QueryCommandRunnerTests.cs #2528 already tracks the observed cdidx index stall on QueryCommandRunnerTests.cs; no duplicate issue was opened.

Fixes #2107