Add snippet and excerpt range metadata

[Widthdom]

Summary

• Add dropped_match_line_count to compact search snippets so clients can detect match lines omitted by the snippet window.
• Add highlights[].term_occurrences with per-occurrence line/column/length data while preserving the existing distinct terms list.
• Add semantic_tokens to excerpt --json with lightweight 1-based ranges and token types.
• Preserve C# verbatim identifier coordinates by mapping normalized matches back to raw snippet text.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SearchSnippetFormatterTests|FullyQualifiedName~QueryCommandRunnerTests.RunExcerpt_JsonClampsLongSingleLineContentAroundFocus" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln --no-restore
• Adversarial review pass 2: No blocking/actionable issues found.

Documentation and Changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog fragments:
  • changelog.d/unreleased/1746.added.md
  • changelog.d/unreleased/1773.fixed.md
  • changelog.d/unreleased/1775.fixed.md

Fixes #1775
Fixes #1773
Fixes #1746