Fix index batch recovery marker

[Widthdom]

Summary

• Stamp codeindex_meta.batch_in_progress=true before index write batches and clear it inside the committing transaction.
• Warn and demote readiness bits on the next writable DB open when an orphaned batch marker remains.
• Cover committed vs rolled-back marker behavior with DB tests and document the recovery contract.
• Fix the CI-blocking Rust associated type default extractor regression found after merging latest origin/main.

Fixes #2109
Fixes #2609

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -p:UseSharedCompilation=false --filter "FullyQualifiedName~DatabaseTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -p:UseSharedCompilation=false --filter "FullyQualifiedName~IndexCommandRunnerTests.Run_FullScan_CancelledAfterReadinessDemotion_RollsBackExistingIndex"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -p:UseSharedCompilation=false --filter "FullyQualifiedName~SymbolExtractorTests.Extract_RustTraitAssociatedTypeDefaults_RecordsPropertySymbols|FullyQualifiedName~DatabaseTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~DatabaseTests|FullyQualifiedName~IndexCommandRunnerTests.FullScanInterruptedDuringWritePhaseRollsBackReadinessDemotion|FullyQualifiedName~SymbolExtractorTests" (adversarial review session; 1112 passed on net8.0 and net9.0)
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check

Documentation and Changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/2109.fixed.md.
• Added changelog.d/unreleased/2609.fixed.md.

Review

• Adversarial review found stale readiness bits were still trusted after warning; fixed by demoting PRAGMA user_version on writable open when batch_in_progress=true.
• CI follow-up found the interrupted full-scan rollback test still expected the old readiness contract; updated it to assert readiness demotion and warning.
• Pipeline follow-up found Rust trait associated type defaults were still scanned under interface containers even though Rust traits are protocol; fixed under Rust associated type defaults are skipped under trait containers #2609.
• Latest adversarial review found only that Rust associated type defaults are skipped under trait containers #2609 was uncommitted; fixed by committing it.

Follow-up candidates

• None.

Notes

• Full dotnet test CodeIndex.sln -p:UseSharedCompilation=false was attempted but stopped after several minutes without progress while other worktree test runs were also active in the environment.