Timer Scheduler

Use a min-heap of monotonic deadlines and generation-tagged timer records. A dispatcher condition loop claims due callbacks under lock and invokes them outside the lock with explicit cancellation semantics.

1. Schedule creates an active timer and pushes (deadline, insertionSequence, id, generation). Equal deadlines follow insertion order; relative delays become monotonic deadlines once.
2. Reschedule increments generation and pushes a new entry. Cancel transitions an unclaimed active record to canceled. Wake the dispatcher whenever the effective earliest deadline may change.
3. The dispatcher skips stale entries, waits until the live earliest deadline and atomically claims a due callback. Cancel after claim returns TooLate. The callback runner invokes outside the lock and reports exceptions separately.
4. Shutdown stops new admission, wakes waits and cancels pending timers under this policy. Claimed callbacks finish/cooperatively stop; obsolete heap entries are periodically compacted to bound memory.

• An earlier insertion wakes and retargets the waiting dispatcher.
• Canceled/rescheduled generations cannot fire stale callbacks.
• Equal deadlines follow the specified tie order.
• Callbacks can reschedule safely and errors/shutdown do not corrupt the heap lifecycle.

Trade-offs to explain

• Lazy cancellation keeps mutation simple but requires compaction for many obsolete entries.
• One synchronous callback thread gives ordering but a slow callback delays later timers; parallel runners change execution-order semantics.
• A monotonic clock measures elapsed scheduling time and does not directly express recurring wall-clock schedules.