fix(registry): recover incomplete generation cleanup

[yaojin3616]

Summary

• tolerate generation directories whose generation.json disappeared during a partial Windows deletion
• discover unreferenced directories independently of their metadata while retaining fail-closed validation for desired generations
• move unreferenced generations from live to uniquely named trash entries before recursive deletion
• cover incomplete metadata, desired-generation protection, and locked-child retry behavior

Verification

• npm test -- --run test/generation-registry.test.ts test/generation-projection.test.ts test/generation-launch.test.ts (34 tests passed)
• npm test -- --run (854 tests passed)
• git diff --check
• npm run typecheck (blocked by pre-existing test/desktop-client-ui.test.ts:108-109 TS18048 errors on origin/main)

Fixes #389