Add status and index diagnostics

[Widthdom]

Summary

• Add DB/WAL sizes and per-language symbol-kind histograms to status JSON.
• Add current process heap/GC/working-set metrics and persisted last-index-run metadata for CLI/MCP index runs.
• Add index --memory-trace JSON memory timelines, CDIDX_MEM_WARN_MB warnings, docs, tests, and changelog fragment.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedNameCliFlagSchemaTests|FullyQualifiedNameDbReaderTests.GetStatus_ExposesOperationalMetrics|FullyQualifiedNameJsonOutputSnapshotTests.RunStatus_JsonOutput_MatchesGolden|FullyQualifiedNameCliJsonSerializerContextTests|FullyQualifiedName~JsonOutputApiVersionTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Release/net8.0/cdidx.dll status --json
• dotnet ./src/CodeIndex/bin/Release/net8.0/cdidx.dll index . --files README.md --json --memory-trace --parallelism 1
• codex adversarial review: No blocking/actionable issues found.

Notes

• Full local index rebuild still hits the existing extraction-stall issue tracked by Full index can stall on current repository after branch refresh #2717, so validation used targeted index refreshes.

Fixes #1637
Fixes #1680
Fixes #1792
Fixes #1817