feat: AP scheduler entry (SMP Phase 3)

feat: AP scheduler entry (SMP Phase 3)

Enable scheduling on Application Processors:

- Load IDT on APs via idt_load_ap() — root cause of AP crashes was
  missing lidt, causing triple-fault when LAPIC timer fires
- Create per-CPU idle process for each AP in sched_ap_init()
- Start LAPIC timer on APs using BSP-calibrated ticks (no PIT
  recalibration needed — all CPUs share same bus clock)
- AP timer handler calls schedule() for local CPU runqueue
- BSP signals APs via ap_sched_go flag after timer_init completes
- Allocations in sched_ap_init done outside sched_lock to avoid
  ABBA deadlock with heap lock
- TSS updates restricted to CPU 0 (shared TSS, only BSP runs
  user processes)
- AP stack increased to 8KB to match kernel thread stack size

All processes still assigned to CPU 0 — Phase 4 will add load
balancing to distribute processes across CPUs.

83/83 smoke tests pass (8s), cppcheck clean.