GC: parallelize and shorten shared precise collection pauses

[chrisbbreuer]

Parent: #459
Depends on: #461

Goal

Reduce shared precise-GC pause time and collector CPU by making sweep/reclamation and safe phases parallel/incremental where provably correct.

Measured problem

The July 29 moving shared row reaches 110.56 ms maximum pause, 0.80x non-moving-parent throughput, and bounded rendezvous retries. The earlier object-churn profile attributes 87.4% of its eight-lane cooperative pause to nursery sweep, while rendezvous and trace are negligible. A whole-run backing-reclamation prototype reduced sweep only 0.7% and regressed wall time 0.9%, so that rejected design must not be repeated without new evidence.

Scope

• Re-profile the current precise nursery after 14d94ec0 and separate park, root publication, prepare, trace, copy/rewrite, sweep, finalization, backing reclaim, and resume.
• Partition sweep/reclamation by independently owned slab/chunk/card ranges with deterministic merge and no shared hot cursor.
• Move safe bookkeeping outside stop-the-world windows or make it concurrent/incremental with explicit phase/state barriers.
• Bound helper count, work stealing, scratch, retry, and tail latency; avoid parallel overhead on small heaps.
• Coordinate all algorithm changes with the owned ../zig-gc sibling and keep stable-ID/relocation contracts exact.

No-workaround rules

• No longer allocation tranche presented as a collector fix, skipped sweep/finalization, hidden whole-run retention, disabled movement, larger unbounded reserve, or lower collection frequency without equivalent memory evidence.
• Do not reintroduce the rejected Perf: reclaim dead nursery backing in whole slab runs #427 whole-run scheme unchanged.

Pre-registered objective gate

Before the first production optimization lands, run the current clean exact-parent phase/matrix harness and check in or attach a frozen objective artifact naming the targeted rows, pause p50/p95/max, wall/CPU, phase times, helper/scratch/RSS, retry/timeout, and required non-regression rows. Success thresholds and allowed noise are fixed before candidate results are inspected; a methodology correction versions the baseline and preserves the original.

Acceptance

• Current exact-parent profiles show the post-change phase responsible for the gain; checksums and byte conservation match.
• The candidate satisfies the pre-registered objective artifact without moving thresholds, deleting rows, or changing work.
• Shared pause p50/p95/max and wall throughput improve materially with bounded scratch/RSS and no single-mutator regression.
• Ephemeral, mixed, high-survival, shared, compaction, weak/finalization, fault/OOM, and movement inventories pass.
• Mid-GC fuzz, lifecycle fuzz, no-GIL corpus, full unit, and suppression-free TSan pass at production time budgets.
• Raw dated evidence is published only after a clean full matrix.

Foundation

#426, #427, #446, #447; current generation evidence; precise nursery batching at 325a12ba.