Fix indexing liveness for slow extraction

[Widthdom]

Summary

• add phase-qualified JSON liveness for update and full-scan indexing, including parallel worker extraction
• bound the C# type-expression regexes used during reference extraction and report regex timeouts with a safe per-file message
• add regression tests for timeout message formatting and JSON heartbeat phase selection

Validation

• dotnet build CodeIndex.sln
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexCommandRunnerTests.Format|FullyQualifiedName~IndexCommandRunnerTests.GetJsonIndexHeartbeatPath" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexCommandRunnerTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --rebuild --json --yes
• Codex adversarial review: No blocking/actionable issues found.

Changelog

• changelog.d/unreleased/2627.fixed.md

Fixes #2627