Fix scan edge cases and canonical indexing

[Widthdom]

Summary

• Fix directory traversal to skip already visited symlink targets and cap unexpectedly deep scans.
• Compute checksums from canonical indexed content after line-ending and line-leading invisible normalization.
• Skip real Git LFS pointer placeholders from body indexing and report lfs_pointer_skipped, while preserving normal text that only mentions the LFS version line.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~FileIndexerTests" -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false passed before the latest origin merge; after the latest origin merge, net8 passed and net9 failed 77 tests with System.ObjectDisposedException: Cannot write to a closed TextWriter in unrelated console/MCP test paths.
• Adversarial review: No blocking/actionable issues found.

Related

• net9 full test run can fail with closed TextWriter during parallel console writes #2775

Fixes #1466
Fixes #1467
Fixes #1622