perf(gc): whole-block in-place promotion — kill the ~243 ns/object promotion tax on a fully-live nursery

[proggeramlug]

retain (3M records, none ever dropped) and retain_wide are the only two
benchmarks still far off Node — 0.80 s vs 0.12 and 1.33 s vs 0.15 on the pinned
M1 mini. Everything else is within 1.2×–3.6×.

The cause is that every survivor gets promoted, object by object. The GC
trace shows 2,097,155 promotions across four copying minors at 509.7 ms of
pause — 243 ns per object — spent on arena_alloc_gc_old, the memcpy,
layout_transfer, old_page_account_promoted_object, the GcMoveHookKind
hooks, and the forwarding/rewrite that follows. The measured young-survival
ratio on those cycles is 1.000: not one of those objects had a reason to
move.

The structural answer is V8-style whole-page promotion: when a block is
(near-)entirely live, relabel the block as old-gen instead of evacuating it.
Nothing moves, so there is no copy, no old-gen allocation, no layout transfer,
no move hook, no forwarding pointer, and no slot anywhere in the heap or in any
address-keyed side table needs rewriting.

Things this has to get right:

• The promotion decision. Block liveness is not knowable before the trace
  and Eden blocks are recycled at offset 0, so per-block history means nothing.
  A per-cycle decision from the previous cycle's measured survival ratio works
  only if a promoting cycle still measures.
• Page-generation metadata and the remembered set when a page changes
  generation in place — this is where correctness lives. In particular
  Old ⟹ TENURED (perf(gc): write barriers cost 16% on an all-numeric store workload — elide on provably-non-pointer stores #7511) and the old-gen page-object index, which is what the
  remembered-set dirty scan uses to reach an object at all.
• Pacing. A copying minor recycles Eden's blocks; promotion hands them away.
  The arena-bytes trigger fires on arena_total_bytes(), so the allocation
  runway collapses unless the capacity is given back.
• Footprint. Dead objects on a promoted page are retained until the next
  full. The per-cycle cost is (1 − liveness) × young bytes; the steady state
  needs its own bound.

[proggeramlug]

Measurement of the residue named in #7744's "Not closed", on main @ 0a2bf15 (perry-dev, M1 Max), retain.ts under PERRY_GC_TRACE=1.

The one full mark-sweep is 36% of all GC pause, and its premise does not hold on this workload.

6 cycles, 504.4 ms total pause. Five minors; one full at 183.7 ms.

What that full did:

arena in-use before → after	67.0 MB → 59.0 MB
reclaimed	8.37 MB = 11.9%
retained_forwarded_stub_bytes	0
retained_forwarded_stub_objects	0

Where its time went: trace_worklist 93.1 ms (55%), atomic_finalize 32.9 ms (19%), remembered_set_marking 28.9 ms (17%), sweep 9.5 ms (6%).

So more than half of it is re-tracing the 3M-object live worklist to free 8 MB, and it freed zero forwarding stubs — which is the entire documented justification for the arena-growth escalation ("a non-moving minor sweep cannot free array-growth forwarding stubs … only a FULL mark-sweep reclaims them"). Being under the 20% productive threshold, it is also what pushes GC_MAJOR_PACING_BACKOFF_SHIFT 0 → 1, i.e. the yield-adaptive backoff (#7726) only learns the bet was bad after paying 183.7 ms for it, once per run.

The signal is available before escalating. set_forwarding_address has exactly two production callers: array/push_pop.rs:162, which leaves the retained array-growth stub, and gc/copying.rs:744, whose forwarding is transient and cleared by the same cycle. So retained stub debt is countable with one add on the growth path and a reset when a full sweeps them, rather than the object walk evacuation_policy uses today. arena_growth_full_escalation_due could then require that there is stub debt to reclaim, instead of discovering there was none by doing the work — and old-gen garbage is unaffected either way, since old_reclaim_pressure_due is a separate trigger.

Note this got cheaper to justify after #7744: whole-block in-place promotion does not copy, so it does not leave forwarding stubs at all on exactly the high-survival workloads where the escalation is most expensive.

I have not implemented this — it is a cross-cutting collector change that needs the bench corpus and a gc_ratchet pass to land safely, and I did not want to start it inside an unrelated PR. Posting the measurement so whoever picks it up starts from numbers.

(The other half of the residue #7744 names — the remembered-set scan over the array's dirty element slots — is untouched by the above.)