Cap per-file symbol output during indexing

[Widthdom]

Summary

• Add cdidx index --max-symbols-per-file N to cap pathological per-file symbol output.
• Persist symbol_count_exceeded file issues and skip content/chunk/symbol/reference rows for files above the cap.
• Make unchanged-file reuse reindex previously capped files when the cap is raised, and improve extraction-stall recovery guidance.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~IndexCommandRunnerTests.Run_FileAboveMaxSymbolsPerFile_PersistsSymbolCountExceededIssueOnly /p:WarningsNotAsErrors=IL2026%3BIL2067%3BIL2072%3BIL2075%3BIL3050
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --rebuild --yes --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Full dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj /p:WarningsNotAsErrors=IL2026%3BIL2067%3BIL2072%3BIL2075%3BIL3050 currently fails in the unrelated trimmed publish test path because test-internal dotnet publish still treats IL3050 as an error. Tracked separately in #2801.

Documentation And Changelog

• Updated README.md for the new indexing cap flag and stall remediation hint.
• Added changelog fragments:
  • changelog.d/unreleased/1604.fixed.md
  • changelog.d/unreleased/2744.fixed.md

Follow-Ups

• See Trimmed publish tests fail on IL3050 AOT analyzer warnings #2801 for the unrelated IL3050 trimmed publish test failure found during validation.

Fixes #1604
Fixes #2744