Fix excerpt semantic token coordinates

[Widthdom]

Summary

• Return excerpt --json semantic tokens in source coordinates and add semantic_token_coordinate_space plus content_line_spans so clamped content can be mapped back to the original source.
• Exclude clamp marker text from semantic tokenization and keep MCP output-budget truncation from returning stale excerpt spans/tokens.
• Document the JSON contract in both English and Japanese and add a changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~RunExcerpt_JsonClampsLongSingleLineContentAroundFocus" -p:UseSharedCompilation=false
• UPDATE_SNAPSHOTS=1 dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --no-restore --filter "FullyQualifiedName~JsonOutputSnapshotTests.RunExcerpt_JsonOutput_MatchesGolden"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --no-restore --filter "FullyQualifiedName~RunExcerpt_Json"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ApplyExcerptOutputBudget_TruncatesAtLineBoundary_Issue1605" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter "FullyQualifiedName~QueryCommandRunnerSearchTests|FullyQualifiedName~McpServerTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• dotnet build CodeIndex.sln --no-restore -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Scope

• DbSearchReader should avoid full line materialization for snippet generation #3507 was not changed because it already had a work-started comment.

Fixes #3563