Optimize indexed test suite performance

[Widthdom]

Summary

• add a batched indexed-file fixture helper that preserves transactional behavior while deferring repeated aggregate refreshes
• apply the helper across high-cardinality C#, Python, TypeScript, Ruby, XML, Rust, Kotlin, Java, JavaScript, YAML, batch, SQL, and F# query fixtures
• consolidate related read-only CLI regression cases into shared immutable indexed workspaces while retaining their existing assertions
• remove 172 redundant explicit SQLite pool clears before the existing failure-driven directory cleanup path

Impact

The full Release test suite completes substantially faster without changing production behavior:

• net8.0: 31m08s to 26m25s (15.2% faster)
• net9.0: 29m04s to 23m46s (18.2% faster)

The test count decreases by nine because thirteen separately indexed cases are represented by three consolidated tests, with one new helper regression test added. The original behavioral cases and assertions remain covered.

Validation

• Release builds for net8.0 and net9.0: 0 warnings, 0 errors
• full net8.0 suite: 10,996 passed, 7 skipped, 0 failed
• full net9.0 suite: 10,537 passed, 405 skipped, 0 failed
• focused helper, shared-workspace, search guard, and fixture lifecycle tests passed on both target frameworks
• separate adversarial review: No blocking/actionable issues found.

Documentation and changelog

• updated the English and Japanese guidance in TESTING_GUIDE.md
• no changelog fragment is included because this changes internal test implementation and contributor guidance only; runtime and user-visible behavior are unchanged

Follow-up candidates

None identified.