feat(park): scheduler coordination layer — parkers, idle mask, wake protocol (RFC 018)

Schedulers get an IO-agnostic sleep/wake primitive of their own: one
futex Parker per scheduler thread (permit semantics, std::thread::park
shaped — closes the check-then-park race), an AtomicU64 idle mask with a
set-bit → re-check → wait park protocol, wake_one (highest-bit LIFO,
CAS-clear before unpark: exactly one wakeup per call by construction),
wake_all for the terminal path, and the timekeeper role — at most one
parked scheduler holds the timer deadline, with an atomic armed-deadline
snapshot for the busy-path due-check and an insert-side re-arm wake.

Deadlines travel as nanosecond timespecs end to end; the wake pipe's
as_millis truncation is unrepresentable here. The Dekker publish/re-check
shape is resolved by the same-location-RMW handshake (AcqRel), not SeqCst
loads; loom verifies exactly this in four models (no-lost-wake, chain
propagation, timekeeper handoff, termination), run with
LOOM_MAX_PREEMPTIONS=3 — unbounded exploration is impractical for the
looped models. Loom/non-Linux builds park on a Mutex+Condvar via
sync_shim.

Standalone until the runtime swap (next commit): nothing outside tests
constructs a Coordinator yet, hence the temporary dead_code allow in
lib.rs.

src/sync_shim.rs

@@ -11,6 +11,15 @@ pub(crate) use loom::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
 #[cfg(not(loom))]
 pub(crate) use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
 
+// park.rs condvar-parker (loom + non-Linux builds only; the Linux non-loom
+// build parks on a futex and never touches these — gating them identically
+// keeps the default build free of unused imports).
+#[cfg(loom)]
+pub(crate) use loom::sync::{Condvar, Mutex};
+
+#[cfg(all(not(loom), not(target_os = "linux")))]
+pub(crate) use std::sync::{Condvar, Mutex};
+
 /// `UnsafeCell` with loom's `with`/`with_mut` access API; pass-through cost
 /// is zero in normal builds (`#[inline]`, newtype over std's cell).
 #[cfg(loom)]