[Bug]: orcad build's @parcel/watcher smoke gate fails good builds roughly 1 run in 3 #17845

Operating system: Linux (Synology DSM)
Orca version: built from main at 94e758666 — between v1.4.191 and v1.4.192

smokeLoadWatcherChild in config/scripts/build-orcad.mjs fails good builds about
one run in three on real hardware, more often the slower the machine.

The gate resolves on the forked child's exit event and treats a non-zero code as
a load failure. But the parent disconnects the IPC channel the instant the child
acks subscribe-started. A child that writes once more then takes EPIPE on an
unhandled 'error' event and exits non-zero — read as "@parcel/watcher failed to
load". Nothing is wrong with the build; the verdict was already in.

Repro:

1. Run the orcad build on a slow machine (consistent on a Synology NAS, rare on
   fast hardware).
2. Roughly 1 run in 3 fails at the watcher smoke step.
3. Re-run with no changes — it passes.

The ack is the real verdict: it is emitted once the child's module graph has
resolved under plain Node, which is all this gate claims to prove. It runs
deliberately before the native module is touched, so a build machine with no
compiled @parcel/watcher still passes. Past the ack the child's fate is not
evidence. An exit before the ack is still a genuine failure.

A gate that fails good builds at this rate is worse than none — it teaches
everyone to re-run, and then it cannot catch the real failure either.