* test(#4105): guard the #3889 hang fixture — must genuinely hang by itself and self-terminate
RED at this sha: against the current never-settling-promise body the guard
fails on the matrix line (Node 24: the unheld promise never self-terminates,
ceiling expires) and off it (v22-class runtimes: the child exits rc=1 after
~60ms, never reaching the still-hanging checkpoint). Same shape as the #4104
self-exit regression: spawn the exact served body, observe liveness past the
chunk bound and natural exit — no elapsed-value assertions.
* fix(#4105): park the #3889 hang fixture on a settling timer
The never-settling promise held no libuv handle, so the hang T1/T4 rely on
was a property of the runtime's test-runner shutdown behavior, not of the
fixture: v24/v26 happen to hold the loop open; v22-class runtimes exit rc=1
after ~60ms (# cancelled 1), so the chunk never reaches the timeout path and
the two timeout assertions assert nothing. Park on a settling 10s timer (the
#4104 idiom): an explicit handle makes the hang the fixture's on every Node
line, 10s >> the 2000ms chunk bound (margin asserted structurally in the
#4105 guard), ~0% CPU while parked, and guaranteed self-termination if a
kill orphans it. Behavior on the Node 24 matrix line is unchanged — the
chunk is still killed by the harness timeout (~2006ms) with the identical
diagnostic.
* test(#4105): drive the fixture guard off the child's exit event + runner timeout
Review-driven restructure (Memtrace flaky_test_fixed_sleep on the 200ms poll
interval): the guard now waits on the child's natural 'exit' event — no
polling interval, no hand-rolled watchdog setTimeout. The immortal-body bound
is the node:test per-test { timeout: 2 * HANG_PARK_MS } backstop, the
health-validation #663 house pattern and the no-elapsed-assertion-compliant
form. t.after still reaps the child on every path. Same failing-first arms:
still-hanging checkpoint, natural-exit (no signal), exit code 0.
* changeset(#4105)
* changeset(#4105): backfill PR number
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Co-authored-by: sim <sim@local>
