Add HTML data and ARIA attribute symbols

[Widthdom]

Summary

• Index HTML data-* and aria-* attribute names as property symbols.
• Normalize those attribute symbols to lowercase while anchoring them at the attribute name line.
• Add focused HTML symbol extractor coverage and an unreleased changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net9.0

Note: dotnet test CodeIndex.sln was also attempted, but the multi-target run failed in the existing trimmed publish test because net8.0 and net9.0 concurrently wrote the same publish output. The same suites passed when run per target framework.

Documentation and changelog

• Added changelog.d/unreleased/2106.added.md.
• No README or guide changes were needed because this is an extractor coverage addition for existing symbol output behavior.

Review

• Manual adversarial review of origin/main..HEAD: No blocking/actionable issues found.
• codex exec adversarial review was attempted, but the Codex CLI returned a usage-limit error before reviewing.

Fixes #2106