Under display.busy_input_mode: queue, sending two messages back-to-back
hung the session on 'Analyzing…' until a manual Ctrl+C.
The submit path only marked the session busy inside the .then of an
async input.detect_drop RPC. dispatchSubmission routes queue-vs-send on
getUiState().busy, so a second Enter inside that RPC window read
busy===false and raced a second prompt.submit down the send path
instead of enqueuing locally. The gateway accepts the mid-turn submit
as a success ({status:'queued'}, not an error), and the client's only
re-queue recovery is gated on catching a 'session busy' error — which
never fires — so the message became invisible to the client-side drain
effect and the UI stayed busy forever.
Extract the ready-prompt submit into a pure submissionCore module and
mark the session busy synchronously at the choke point, before the
detect_drop round-trip, closing the gap for every caller (mainline
submit, queue-edit picks, drain, interpolation). Verified the real
gateway already queues+drains both turns correctly, so the fix is
purely client-side. Adds submissionCore.test.ts whose regression
assertions fail without the synchronous busy and pass with it.
e71f9ad0bb