Player::pause_acked() returns a generation-bound PauseAck applied on the
audio thread; a desired-paused latch makes an in-flight decoder come up
paused instead of auto-starting, closing the window where a pause lands
during decode setup. Proven live by examples/focus_ack_probe.rs:
ack-while-playing freezes position; ack-while-buffering never becomes
audible; resume clears the latch; plain pause() during buffering is
demonstrated to auto-start (pre-existing hazard, filed separately).
Transport ADR recorded (native), Phase 0 marked complete in the plan.
