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
0d88e933e6