Surface oversize file diagnostics

[Widthdom]

Summary

• Persist files that exceed the configured index size limit as file rows with a file_too_large issue instead of treating them only as per-file errors.
• Teach validate --kind about file_too_large and document the new diagnostic path.
• Add regression coverage for full scan and --files oversize indexing paths.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~IndexCommandRunnerTests.Run_FileAboveMaxFileBytes_PersistsFileTooLargeIssue|FullyQualifiedName~IndexCommandRunnerTests.RunFiles_FileAboveMaxFileBytes_PersistsFileTooLargeIssue|FullyQualifiedName~IndexCommandRunnerTests.ParseArgs_MaxFileBytes|FullyQualifiedName~FileIndexerTests.BuildRecord_LargeFile" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• adversarial review: No blocking/actionable issues found.

Documentation and Changelog

• Updated USER_GUIDE.md.
• Added changelog.d/unreleased/1603.fixed.md.

Follow-up Candidates

• Local .cdidx refresh was interrupted while reindexing large repository files in this workspace; the PR validation above uses build/test outputs rather than relying on that local index state.

Fixes #1603