fix(scheduler): align(64) on SchedulerStats to prevent false sharing

Adding slot_hits + slot_displacements in RFC 005 grew SchedulerStats
from 16 to 32 bytes — exactly 2 per cache line. run_queue_len (written
on every push/pop by thread N) then false-shared with fields of thread
N+1, causing ~45-60% yield-storm regression at t≥8 visible equally in
slot-on and slot-off paths.

src/runtime.rs

@@ -275,6 +275,7 @@ impl Default for Config {
 
 /// Lockless per-scheduler-thread counters. Written only by the owning thread;
 /// readable from any thread (introspection actor, tests).
+#[repr(align(64))]
 pub struct SchedulerStats {
     /// PID index of the actor currently on-CPU, or `u32::MAX` when idle.
     pub current_pid_index: AtomicU32,