Move the identical in-memory child used by the per-call and persistent
host specs into tests/fake-child.ts, and move readProgramResult next to
the ProgramResult type it validates in protocol.ts.
One pipe delivers each evaluation's messages before its result, and the
child refuses calls from finished evaluations, so the per-message
evaluation id protected nothing. Remove it with the checks, counters and
helpers it required, measure outgoing messages only when posting, and
replace rethrowing validation blocks with early refusals.
Replace the length-prefixed JsonChannel and hand-written request/reply
correlation with typed birpc methods over newline-delimited JSON. The
host calls the child's run(); bindings call the host's invoke(); logs
and output-limit reports are notifications. The transport bounds
incoming lines and queued writes, validates UTF-8 and exposed methods,
and rejects outstanding calls when the pipe closes.
Drop logic made obsolete by the new transport, including the 32-bit
frame-length bound on maxMessageBytes.
_Kerman