Addresses the review's two remaining items:
- The host closes the child's stdin write handle immediately after spawn. The
program is an async body that reads nothing from fd 0; a live pipe would hold
a host-side handle open past the run, so a setsid-escaped descendant
inheriting fd 0 could keep the host process from exiting even after the
closeDeadline forced settlement. The child (and any descendant) reads EOF on
fd 0 and no host handle survives.
- read_frame/read_frame_async bind their decode primitives (_decode_json_plain,
os.read, _READ_CHUNK_BYTES, bytes) as def-time default arguments, and
_decode_json_plain itself captures json.loads, its two regexes, and len the
same way, so a __main__ rebind cannot kill the reply pump and strand every
pending Future to the wall clock. _decode_json_plain and its regexes moved
before the ProtocolChannel class so the defaults resolve at class-definition
time. A regression test rebinds _decode_json_plain and asserts a binding reply
still round-trips.
Note (en + zh) registers both mechanisms; pairings re-recorded.
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