events.once/events.on with AbortSignal: process never exits after completion (event loop parks on a leaked keepalive)

[proggeramlug]

Found while fixing #4633 on main @ 6c85905 (macOS arm64).

Repro

Any program that passes an AbortSignal to events.once/events.on finishes its work, prints the correct output, then never exits — the main thread parks in __psynch_cvwait (event-loop park) until killed:

perry compile test-parity/node-suite/events/once/abort-cleans-pending-error.ts -o /tmp/t && /tmp/t
# correct output, then hangs; node exits 0

Same for test-parity/node-suite/events/on/async-iterator-abort.ts (after the #4633 SIGSEGV fix). Both tests pass the parity harness because it compares timeout-truncated stdout — exit codes hide the hang.

Notes

• Pre-existing on main: once/abort-cleans-pending-error has been passing parity while hanging at exit (exit 124 under timeout 8).
• Suspects: a pending JsPromise keepalive (events.on pre-allocates an abort_promise that is re-pushed onto the queue tail per emit in events_on_queue_listener), or the AbortSignal listener registration counting as an active handle even after js_abort_signal_remove_listener.
• sample: main thread blocked in _pthread_cond_wait under two unsymbolicated perry frames (release build).

[proggeramlug]

Verified on current main (86bf07b51, macOS arm64, Node reference v26.3.0): this no longer reproduces — both programs exit 0.

$ perry compile test-parity/node-suite/events/once/abort-cleans-pending-error.ts -o /tmp/t && /tmp/t
abort: AbortError:ABORT_ERR
late error: late
# exit 0 (was: hung in __psynch_cvwait until killed)

$ perry compile test-parity/node-suite/events/on/async-iterator-abort.ts -o /tmp/t2 && /tmp/t2
abort: AbortError ABORT_ERR
seen: a,b
# exit 0

Both match node --experimental-strip-types byte-for-byte (output and exit code 0). I ran the once case 5× and the on case 3× — exit 0 every time, no park at exit.

The fix landed between the commit this was filed against (6c85905cc) and now; the only change to events.rs/abort/event-loop in that range is #5099 (48f0935ea, "events.on async iterator … node v26 parity"), which reworked the events.on async-iterator path and evidently cleared the leaked keepalive.

Recommend closing as resolved. (The orthogonal harness gap noted here — node-suite comparing timeout-truncated stdout and ignoring exit codes, so an exit-hang passes parity — is real but separate; it's covered by the node-suite regression-guard work and not specific to events.)