Keep failed-member exclusion across the room queue, rather than resetting
it per pending thread. A new user action after failure still permits a new
attempt. Share the drain activity epoch so a skipped queued thread cannot
hide the preceding member failure.
Proven red in real Electron: hold transport refusal, enqueue same-thread
and cross-thread sends, then release; old head submits three times, fixed
head once. Strengthen follow-up evidence with distinct provider replies,
exact public log order/count, and per-input inference counts.
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