GC: make the multi-age nursery moving

[chrisbbreuer]

Parent: #134
Extends: #145
Depends on: #144
Related: #446
Blocks: #147, #246

The production age-three nursery currently reclaims and promotes without relocating surviving young cells. The README removal gate requires a moving nursery whose copying/promotion path reuses the exact relocation contract and remains safe during mid-script no-GIL minor collection.

Acceptance

• Copy surviving young cells between age spaces and promote terminal-age survivors with stable IDs and failure-atomic forwarding metadata.
• Rewrite precise roots, old-to-young remembered edges, young-to-young edges, weak/finalization state, handles, interpreter/baseline/optimizer stack maps, and embedding roots during every moving minor cycle.
• Preserve persistent-card correctness and eliminate stale from-space pointers before mutators resume; verify the full relocation inventory after each stress cycle.
• Coordinate bounded parallel mid-script minor rendezvous across shared-realm mutators without a context GIL, including allocation failure and collector abort recovery.
• Keep conservative stacks and opaque native frames explicitly pinned or fail closed; never infer interior/base pointers from untyped words.
• Add age-by-age promotion, weak/ephemeron/finalizer, JIT deopt/OSR, native handle, fault-injection, leak, no-GIL, TSan, and long-running mutation tests.
• Publish exact-parent pause, throughput, bytes-copied, promotion, retained-backing, and timeout evidence; update the release matrix and README only from committed artifacts.