GC: finish parallel collections in mid-GC condition and ThreadLocal witnesses

[chrisbbreuer]

Parent: #476. Related: #502 and #792 (distinct failure signature).

Linux CI on f2340f2 failed the non-TSan
threadfuzz-midgc gate:
https://github.com/zig-utils/zig-js/actions/runs/33137549779/job/98740893155

Command: THREADFUZZ_SEED_TIMEOUT_MS=300000 zig build threadfuzz -Dfuzz-midgc=true -Dfuzz-iters=20.

Exact functional failures:

• seed 8: midgc Atomics.Condition.wait did not finish a parallel collection
• seed 12: ThreadLocal lifecycle did not finish a parallel collection
• seed 17: midgc Atomics.Condition.wait did not finish a parallel collection

Final denominator: 900 programs from seed 1, 3 failures, wall-ms 931851.
This is a failed collection-completion witness, not a reported sanitizer race,
crash, or the condition completion-score assertion in #792. Root cause and
introducing revision have not been established; do not label it a regression
from parameter lowering without a before/after reproduction.

Reproduce the named seeds, inspect attempted/completed/aborted collection and
peer-publication/parking evidence, and correct ownership or progress at the
responsible boundary. Preserve the existing assertion and work budget: no
added sleeps, relaxed denominator, forced extra drain, suppressed/disabled GC,
or weakened collection requirement. Coordinate any native wait/job/lifecycle
change with the actor owning that work. Verify normal and unsuppressed TSan
mid-GC gates plus the full no-GIL integration checks.

[chrisbbreuer]

Environment confirmed from the completed job log: ubuntu-24.04 x64, Zig 0.17.0-dev.1818+7051f8e73. Local macOS builds use 0.17.0-dev.1441_d5181a9c9, so a passing local seed alone would not establish Linux parity. The complete failed-job log is retained in /Users/chris/.cache/zig-js-proxy.Hdh6eC/prior-ci-midgc-job.log.