fix: connect engine in v0.29.1 migration

[alexandreroumieu-codeapprentice]

Summary

• Connect the migration-created engine before using it in v0.29.1 backfill and verify phases.
• Disconnect the engine in finally to release PGLite resources/locks.
• Add a PGLite regression test covering phaseBBackfill and phaseCVerify via a temporary GBRAIN_HOME.

Root cause

phaseBBackfill and phaseCVerify called createEngine(...) but then used the returned PGLite engine without first calling engine.connect(engineConfig), causing:

PGLite not connected. Call connect() first.

Validation

• bun test test/migration-v0-29-1.test.ts
• bun run typecheck

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