Pre-flight for Phase 0c. The P8-percpu-sched and P8-percpu-wiring
patches both reference crate::context::RUN_QUEUE_COUNT but none of
the kernel P5–P9 patches define it (verified by grep). The downstream
patches have an incomplete dependency: they need this constant at
the module level but no patch supplies it.
Add 'pub const RUN_QUEUE_COUNT: usize = 40;' here, matching the
historical 40-priority DWRR queue count. The P7-cache-affine-context
patch separately defines 'pub const SCHED_PRIORITY_LEVELS: usize = 40;'
in context/context.rs which is a duplicate; both being 40 keeps the
existing SCHED_PRIO_TO_WEIGHT and quantum tables valid.