Isolate unreadable marker fixtures across target frameworks

[Widthdom]

Summary

• Move the unreadable MCP marker-fingerprint fixture to a unique temporary workspace outside repository and target-framework output trees.
• Temporarily scope the process current directory to that workspace so MCP path authorization remains valid, then restore it in finally.
• Document the fixture-isolation contract in both English and Japanese and add a bilingual changelog fragment.

Root cause

The fixture previously lived beneath the test assembly's current directory. During a default multi-target run, one target framework could recursively enumerate another target's bin tree while the latter intentionally held its secret directory unreadable. Filtering bin / obj after recursive enumeration was too late to prevent the access failure.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore
  • succeeded with 0 warnings and 0 errors
• Focused unreadable-marker and source-policy tests under net8.0 and net9.0
  • passed, including an adversarial concurrent cross-target run
• dotnet test --no-restore --nologo -p:UseSharedCompilation=false
  • net9.0: 10,622 passed, 420 skipped, 0 failed
  • net8.0: 11,095 passed, 7 skipped, 1 unrelated pre-existing timeout; tracked in Investigate net8 parallel batch detached-snapshot test timeout #5033
• dotnet run --project tools/CodeIndex.Changelog -- check
  • validated all 105 fragments
• cdidx status --check --json
  • workspace/index matched; graph data complete
• Codex adversarial review against the final origin/main..HEAD
  • No blocking/actionable issues found.

Documentation and changelog

• Updated TESTING_GUIDE.md in English and Japanese.
• Added changelog.d/unreleased/5019.internal.md.
• Did not edit CHANGELOG.md.

Follow-up

• Investigate net8 parallel batch detached-snapshot test timeout #5033 tracks the unrelated net8.0 parallel-batch detached-snapshot timeout observed during the full test run.

Fixes #5019