kernel: apply P5-context-mod-sched, P8-percpu-sched, P8-percpu-wiring

Phase 0c, plan orders #3, #4, #7.

  P5-context-mod-sched: re-export SchedPolicy from context::mod (one-line
    change to the use statement). The type is defined in context::context
    by the previous P7-cache-affine-context commit; this just makes it
    available as crate::context::SchedPolicy.

  P8-percpu-sched: adds PerCpuSched struct to percpu.rs with SyncUnsafeCell-
    wrapped run_queues, balance/last_queue/last_balance_time cells, and
    take_lock/release_lock methods. Refactors PercpuBlock to embed
    PerCpuSched as 'sched' field instead of standalone 'balance'/'last_queue'
    fields. Adds get_percpu_block() helper.

  P8-percpu-wiring: rewrites src/context/switch.rs to consume PerCpuSched:
    - select_next_context reads from percpublock.sched.queues() instead
      of the global RunContextData.set
    - Initial placement chooses least-loaded CPU via PercpuSched.balance
    - Load balance trigger fires periodically and migrates contexts
      between per-CPU queues respecting sched_affinity
    - Adds pub const fn to access per-cpu sched state safely

After this commit, the kernel builds with per-CPU run queues wired
into the scheduler. cargo check still has 1 pre-existing unrelated
error (src/acpi/fadt.rs:110 type mismatch) that predates the threading
work.

Combined with the P6-futex-sharding commit, this completes the
foundation for Phase 1 (Futex Completeness) and Phase 2 (SMP Scheduling
Quality).

src/context/mod.rs

@@ -22,7 +22,7 @@ use crate::{
 
 use self::context::Kstack;
 pub use self::{
-    context::{BorrowedHtBuf, Context, Status},
+    context::{BorrowedHtBuf, Context, SchedPolicy, Status},
     switch::switch,
 };