Force-sending a queued message (double-empty-enter, or interrupt-mode
submit) flipped busy→false optimistically, so the queue drain raced the
still-unwinding turn: duplicate user bubble, a stray "queued: …" note, and
the cancelled turn's "Operation interrupted…" reply leaking in.
interruptTurn gains `keepBusy`: hold busy until the gateway's real settle
edge (message.complete, suppressed while interrupted), which drains the
queued message exactly once — desktop "send now" parity. The interrupt
paths now queue + interrupt instead of optimistically sending.
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