Fix interrupted rebuild rollback preserving index

[Widthdom]

Summary

• Move the full-scan batch marker into the same rollback boundary as readiness demotion and file writes so interrupted scans preserve the previous healthy index state.
• Stop --rebuild from dropping the existing database before the transactional write phase, while still deleting rows for files that become non-indexable during a successful rebuild.
• Add regression coverage for interrupted full scans, interrupted rebuilds, and rebuild cleanup of formerly indexed binary files.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexCommandRunnerTests.Run_FullScan_CancelledAfterReadinessDemotion_RollsBackExistingIndex|FullyQualifiedName~IndexCommandRunnerTests.Run_Rebuild_CancelledAfterReadinessDemotion_PreservesExistingIndex|FullyQualifiedName~IndexCommandRunnerTests.Run_Rebuild_WhenIndexedFileBecomesBinary_RemovesStaleRow"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits bfe49ab86771948c6885d46035f5dff686dbda6b --json
• adversarial review: No blocking/actionable issues found.

Changelog

• changelog.d/unreleased/2642.fixed.md

Fixes #2642