GC nursery: prototype copying minor collector with wholesale reset

[andrewtdiz]

Parent: #1090
Requested assignee: @andrewtdiz

Problem

Minor GC still pays sweep, age-bump, block-persistence, and reset/deallocation costs. The structural win is copying live young objects and resetting nursery pages wholesale.

Context

Prototype a copying young-generation collector with young from-space/to-space or an evacuation buffer. Minor GC should copy live young roots and dirty-page children, rewrite mutable slots, promote survivors when needed, and reset nursery allocation space wholesale.

Target invariant:

minor GC cost = live young objects + dirty old cards/pages + mutable roots

Verification

• Minor pause scales with live young bytes, not dead nursery allocation volume
• bench_gc_pressure pause time decreases
• object_create and binary_trees do not regress
• perf-comprehensive has no mid-round GC outliers
• Memory stability suite passes in copying-nursery mode

[proggeramlug]

Verified resolved against current main: a copying minor collector is implemented and is the default minor-GC fast path. CopyingNurseryCollector (gc/copying.rs) copies live young roots into to-space, scans dirty pages, rewrites mutable slots, promotes survivors, then resets the nursery wholesale (copying_reset_from_spaces_and_flip), with pause cost scaling to live-young bytes and correct fallback to mark-sweep when ineligible. Closing as resolved.