Repair mixed folded backfill state

[Widthdom]

Summary

• Detect non-NULL folded-key drift when NULL folded values are pending.
• Promote mixed missing/stale state to one full rewrite in CLI and MCP.
• Preserve full-rewrite mode across interrupted promoted rewrites until checkpoints clear.
• Consume and exactly verify completed persisted rewrite checkpoints before later scoped repairs.
• Add CLI/MCP regressions, bilingual testing guidance, and a changelog fragment.

Root cause

The preflight selected a targeted NULL-only backfill whenever missing values were present, so non-NULL stale rows survived and final verification failed. Interrupted promoted rewrites could also leave a full-rewrite checkpoint that a targeted retry ignored, including an end-of-table checkpoint that the CLI previously treated as a no-op.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• BackfillFold Release tests on .NET 8 and .NET 9 (41 passed, 1 existing trimmed-publish skip each)
• Issue backfill-fold can leave NULL folded values after a scoped update #4946 regression tests on .NET 8 and .NET 9 (5 passed each)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Real checkpoint reproduction: completed checkpoint cleared, later scoped NULL repaired in targeted mode, 0 NULL rows/checkpoints remained
• Real repository no-op backfill verified fold readiness
• Codex adversarial review: two rounds, three actionable P2 findings reproduced and fixed

A broader solution test run encountered one unrelated timing-sensitive MCP concurrency failure under shared-host load; the exact test passed independently on both .NET 8 and .NET 9.

Documentation and changelog

• Updated TESTING_GUIDE.md in English and Japanese.
• Added changelog.d/unreleased/4946.fixed.md.

Follow-up candidates

• None.

Fixes #4946