0d88e933e6
Background worker threads used blocking tx.send() on bounded channels. If the consumer stopped draining, threads hung forever with no way to reach the cancel check. Drop-issued cancellation was ineffective. Changes: - Progress messages: tx.try_send() (discard if full, never blocks loop) - Terminal messages (Complete/Error): new send_cancelable<T>() helper using crossbeam select! — sleeps efficiently until send succeeds or cancel arrives - Drop cancellation: tx.try_send() — Drop must never block - spawn_visual_height_rebuild: same fix for its bounded(1) channel - 5 new tests covering full-channel + cancel scenarios