preempt: explicit check!() macro for no-alloc loops

Stable Rust emits stack probes inline (subq/movq/jne loop) rather than
calling __rust_probestack, so there's no transparent hook for stack-
frame preemption. Override of __rust_probestack links cleanly but never
runs. Falling back to an explicit check!() that users drop into hot
compute loops.

check!() decrements the same ALLOC_COUNT counter as the heap path, so
both event sources fire timeslice checks at the same rate. Documents
the prep-to-park invariant on maybe_preempt — library code that
registers a wakeup and then parks must keep that window alloc-free and
check-free, or a preemption-driven yield in the middle would lose the
wakeup.

src/lib.rs

@@ -42,3 +42,25 @@ pub use scheduler::{
     block_on_io, run, self_pid, sleep, spawn, spawn_under, yield_now, JoinError, JoinHandle,
 };
 pub use supervisor::Signal;
+
+// ---------------------------------------------------------------------------
+// check!() — explicit preemption point for tight no-alloc loops.
+// ---------------------------------------------------------------------------
+
+/// Voluntarily check whether this actor's timeslice has expired, yielding
+/// if so. Drop this into hot compute loops that don't allocate (heap or
+/// large stack frames) — without it, such loops monopolise the scheduler
+/// until they return.
+///
+/// Decrements the same per-actor event counter as the heap allocator's
+/// preemption hook, so the check rate is identical regardless of whether
+/// the actor is alloc-heavy, check-heavy, or mixed.
+///
+/// No-op outside an actor (the runtime's `PREEMPTION_ENABLED` flag is
+/// false there).
+#[macro_export]
+macro_rules! check {
+    () => {
+        $crate::preempt::maybe_preempt()
+    };
+}