GC relocation: rewrite surviving weak and finalization records

Parent: #345 Depends on: #344 Roadmap: #335, #144, #134

Implement the internal weak/finalization portion after zig-gc has completed ephemeron fixpoint, cleared dead weak slots, pruned entries, and swept dead cells.

Acceptance:

• Clear a FinalizationRegistry target when its record becomes ready so no swept address remains stored.
• Rewrite surviving WeakRef targets, weak collection keys, ephemeron values, finalization callback/held Values, and live unregister tokens.
• Never resolve a dead target through the relocation plan.
• Repair or invalidate pointer-keyed weak indexes.
• Prove surviving/dead/ready ordering and preserve exactly-once cleanup state.
• Focused normal and TSan runs report zero failures and leaks.
• Production movement remains disabled and #345/#335 are updated.

Implemented and pushed in 88dae824.

Delivered:

• FinalizationRegistry records null dead targets as soon as they become ready, before sweep can leave a stale address;
• surviving WeakRef targets, weak keys, ephemeron values, finalization callback/held Values, live targets, and live unregister tokens rewrite after pruning;
• ready/dead targets are never offered to the forwarding plan;
• pointer-keyed weak_index is cleared after key relocation; linear lookup remains correct and later mutations repopulate it;
• exact regression proves live/dead/ready ordering and zero forbidden resolutions.

Local gates:

• filtered normal: 3/3 passed, 0 failed, 0 leaked;
• filtered TSan: 3/3 passed, 0 failed, 0 leaked.

Production movement remains disabled. #345 still tracks native-private and Wasm boundaries.

Completed and accepted. The implementation-specific local evidence is recorded above, and descendant full CI at https://github.com/zig-utils/zig-js/actions/runs/29716788619 completed with 42 successful jobs, one intentional nightly skip, zero failures, and zero cancellations.