The slot epoch is THE wait identity, so the hand-rolled per-primitive copies
go away: channel loses cur_wait/next_wait_seq/timed_out, mutex loses Wait.seq
and next_seq, TimerTarget::on_timeout takes the epoch. Registrations become
(pid, epoch) — channel parked_receiver, mutex waiters, io fd waiters,
Blocking io completions, sleep timers, joiner lists — and their wakers move
to unpark_at. begin_wait is lock-free, so each primitive opens the wait
inside the same critical section that publishes the registration.
recv_timeout's wake-classification loop collapses: wakes are precise, so
queue → Ok, senders==0 → Disconnected, else Timeout — the 'Defensive'
re-register branch is now unreachable by protocol, not by audit. Same for
Mutex::lock_timeout's one-shot park.
End-state invariant, auditable in one sentence: the only wildcard wake is
request_stop, which is terminal.
Adds a BinaryHeap of timer entries on SchedulerState. sleep() inserts
an entry and parks; schedule_loop pops due entries each iteration and
unparks them. When the run queue is empty but timers are pending, the
OS thread sleeps until the soonest deadline.
Single-threaded only; thread::sleep is fine because no other thread
can wake us. The IO thread coming next will need a Condvar or pipe
wakeup to break this OS-sleep early.
smarm