GC: enable automatic compaction at shared precise safepoints

[chrisbbreuer]

Parent: #134
Extends: #144
Related: #145, #433
Blocks: #147, #246

Explicit fail-closed compaction is complete, but the README removal gate still requires production policy that can schedule movement automatically and coordinate it safely with concurrent/shared-realm execution. This issue owns automatic triggering and shared precise stop windows; it does not weaken the existing raw-pointer or conservative-stack fail-closed rules.

Acceptance

• Define a deterministic fragmentation/retained-backing trigger with bounded hysteresis and exact telemetry; no benchmark-source recognition.
• Schedule compaction from ordinary allocation/collection pressure and consume it at every declared precise interpreter, baseline, optimizer, embedding, and shared-realm safepoint.
• Coordinate all live shared-realm mutators and concurrent/minor collectors before movement, with bounded park/abort behavior and no movement under conservative or opaque native frames.
• Preserve stable identity, weak/finalization ordering, handles, inspector/debugger roots, Wasm roots, and exact resumable JIT state across the automatic stop window.
• Make OOM, unsupported stacks, active host callbacks, and concurrent collector conflicts failure-atomic and observable through telemetry rather than silent partial movement.
• Add single-mutator, no-GIL, concurrent-marker, native callback, optimizer, fault-injection, leak, TSan, and long-running fragmentation coverage.
• Publish an exact-parent fragmentation/pause/throughput matrix and update the release compatibility inventory and README only from committed evidence.