The handshake verification used a raw blocking `recv`, the one unbounded
read in the spawn path: `accept` is guarded by `spawn_timeout` and every
host->runner `call` by `call_timeout`, but a process that won the accept
race and then sent nothing — or a runner that died mid-handshake — would
hang the host forever.
Route the handshake through the existing `recv_with_deadline` helper so
the first frame is bounded too (marking the process dead on timeout), and
reduce `verify_runner_handshake` to a pure token check on the received
message. Tests updated to match.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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