GC relocation: rewrite Environment binding graphs

Parent: #335 Depends on: #340 Roadmap: #144, #134

Complete Environment relocation before enabling any cell movement.

Acceptance:

• Rewrite every managed Value in vars, disposables, and dispose_pending.
• Rewrite every live alias Environment, object_proto_intrinsic, parent Environment, and with_object.
• Preserve binding names, declaration sets, allocator/accounting metadata, and arena ownership.
• Relocation runs only at the world-stopped commit boundary and does not acquire binding_lock.
• Distinct-target regressions prove every slot.
• Focused normal and TSan runs report zero failures and leaks.
• Production movement remains disabled and #335 is updated.

Implemented and pushed in b76acb13.

Delivered:

• relocateEnv rewrites every binding Value, disposable value/method, pending disposal value, live alias Environment, realm prototype, parent Environment, and with object;
• binding names, declaration metadata, allocator/accounting fields, and arena-owned containers remain stable;
• relocation is explicitly world-stopped and does not acquire binding_lock;
• distinct-target regression proves every managed slot while preserving alias/name metadata.

Local gates:

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

Production movement remains disabled. Keeping this open only for descendant full GitHub Actions evidence.

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.