Strengthen traversal and cleanup policies

[Widthdom]

Summary

• Add opt-in cancellation and entry-budget support plus shared failure taxonomy to FileSystemTraversalPolicy.
• Revalidate case-sensitivity probe cleanup targets against safe roots, expected names, and symlink/reparse/device status before deletion.
• Document the traversal/destructive cleanup contracts and add an unreleased changelog fragment.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false -m:1 --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --no-restore --filter "FullyQualifiedName~FileSystemTraversalPolicyTests|FullyQualifiedName~CaseSensitivityProbeDirectory_Cleanup"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• destructive-operation and Directory.Enumerate audits with local cdidx search
• Adversarial review: No blocking/actionable issues found.

Fixes #4131