Stabilize C# large-switch performance guard

[Widthdom]

Summary

• widen the dense C# switch-expression regression tripwire from 8 seconds to 15 seconds
• preserve the 10,000-arm fixture and functional symbol assertions while documenting the full-suite intent in English and Japanese
• add a bilingual internal changelog fragment

Root cause

The broad wall-clock runaway guard used an 8-second threshold that behaved like a benchmark under full-suite host contention. In the validated Release/net8.0 full suite, the target test took 9 seconds, while isolated runs took 2–4 seconds. The extraction result remained correct; the failure was caused by insufficient load margin.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• make lint
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• focused Release/net8.0 target test, followed by five repeated runs (all passed)
• Release/net8.0 full suite with repository runsettings and VSTest crash/hang blame: 10,240 total, 10,233 passed, 7 skipped, 0 failed in 32.43 minutes; the target test passed in 9 seconds
• mandatory Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• updated the English and Japanese sections of TESTING_GUIDE.md
• added changelog.d/unreleased/4792.internal.md

Follow-ups

• None.

Fixes #4792