perf(gc): the minor collector still does a full-region sweep walk over the old generation (#6181 sub-cost a)

[proggeramlug]

The one unfixed sub-cost from #6181, refiled on its own so it stops sharing a ticket with two that are done.

Fixed and closed with #6181: (b) the whole-heap remembered-set rebuild and (c) the per-cycle page-meta iterations, via #6213 / #6277 / #6568.

Still live: sub-cost (a) — the minor collector performs a full-region sweep walk over the old generation. crates/perry-runtime/src/gc/oldgen.rs::sweep (line ~476) is reached on the minor path, so minor-GC cost scales with old-gen size rather than with nursery survivors — which is the property a generational collector exists to avoid.

Why this is worth doing and what to watch

The remembered set already exists precisely so a minor need not retrace the old generation, so the walk is redundant work on the hot path. But two things make this less mechanical than it looks:

• The malloc sweep is a separate path (sweep_malloc_objects, copied_minor_malloc_sweep_due) and is gated by trigger kind. Do not conflate the two — a change that skips one and not the other will look like it works.
• A guard alone can livelock. P0: silent wrong answer under the moving collector — stale heap-string deref in generated code (realistic interpreter workload) #7682's fix is the precedent: adding the immobility guard by itself deadlocked, because the nursery cap was keyed on from-space bytes that a non-moving minor never frees. The complete fix needed cap-gating as well (fix(gc): restore evacuation at precise safepoints — the pacing half of #7682 #7690). Expect the same shape here — removing work from the minor path changes what the trigger arithmetic sees.

Before coding

Re-measure. #6181's figures predate #6213/#6277/#6568, so the remaining share attributable to (a) alone has never been isolated. Use PERRY_GC_DIAG=1 and confirm the cycle KINDS you are measuring are the ones you think — census counters cover copying minors ONLY, and a run whose cycles are full mark-sweeps says nothing about the minor path.

Bench host per the standing rule: the quiet M1 mini.