The timer was only armed for states carrying auto_advance_on_silence. Readback
states carry none — they wait for the pilot — so nothing was armed and ATC
never asked again when the readback did not arrive.
Which window applies to a state is now a pure function: the flow-authored one
for an auto-advance state, the much shorter server-published one for a readback
state. That also makes both cases testable instead of inline in the composable.