Remove superseded rebuild finalization

[jyaunches]

Objective

Delete the rebuild finalization implementation that is not called by the rebuild pipeline.

Scope

• Delete src/lib/actions/sandbox/rebuild-finalization.ts and its test.
• Point retired-selector compatibility tests at current rebuild tests.
• Keep rebuild-post-restore-phase.ts as the canonical implementation.

Evidence

The obsolete implementation has no production caller. Its source and test total 355 LOC.

Expected net reduction: 330–380 LOC after any required assertion migration.

Acceptance criteria

• Preserve unique policy, Shields, forwarding, and fail-closed assertions in canonical rebuild tests.
• Compatibility-controller command lists reference only current tests.
• No production rebuild behavior changes.

Validation

Run targeted rebuild source tests, retired-selector E2E-support tests, npm run typecheck:cli, and npm run test:changed.