Function n3_watchdog_tick 

pub fn n3_watchdog_tick()

Called from the timer ISR to check for stalled migrations.

For each registered frame in Active state, checks whether the migration has exceeded the watchdog timeout. If so:

1. Transitions Active => Stalled (CAS)
2. Attempts recovery: Stalled => Reclaiming => Ready
3. Logs the incident

Recovery resets the frame so it can be reused for future migrations. The sender that initiated the stalled migration will see WouldBlock on its next send() attempt (CAS Ready=>Active fails if another sender claimed the frame first).