refactor: remove slab from waiter storage

[tisonkun]

• This closes chore: use Vec instead of Slab to store waiting wakers in WaitSet #103.
• This closes Deregister cancelled broadcast recv futures from WaitSet #115.
• This closes fix(waitset): deregister cancelled broadcast receivers #116.

Summary

• replace the slab-backed waiter stores with a focused internal free-list arena
• simplify WaitList to explicit head/tail links while retaining detached future-owned nodes until poll or drop
• make wake-all registrations epoch-aware and unregister them when waiting futures are cancelled
• reclaim queue-owned Semaphore::forget_exact debt nodes immediately after they are fulfilled
• migrate FairShare to the same arena and remove mea’s only normal dependency

Waiter lifecycle

Future-owned semaphore and condition-variable waiters transition from linked to detached and are removed when their future observes completion or is dropped. Queue-owned permit debt is removed during the same operation that detaches it. WaitSet registrations carry an epoch so a stale registration cannot alias a slot reused after wake-all.

Validation

• cargo x lint
• cargo x test --no-capture
• cargo x bench
• cargo +1.85.0 test --workspace --no-default-features
• cargo tree -p mea --edges normal