FABRIC-3.10.8 MC-3b: waking a worker core

Each worker core sets up its own IPI receiver before going online (amd64
LAPIC + vector 0x22 with its own stub, aarch64 banked GIC SGI 0, riscv64
sie.SSIE); core 0 sends by ICR / GICD_SGIR / SBI IPI. Park lets exactly
one interrupt in without a lost-wake window. Boot self-test: IPI round
trips 3/3 and 'woke x3, IPIs x3' on all three ISAs; worker vCPU threads
0.0% on the host. 3-ISA acceptance clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

include/starkernel/percpu.h

@@ -73,6 +73,10 @@ typedef struct sk_percpu {
     struct VM        *release_after;
     volatile uint32_t decision_due;
     struct VM        *entry_vm;      /* the VM a first-entry trampoline starts as */
+    /* MC-3b (hal/smp.c): how to reach this core with an IPI (the APIC id,
+     * the GIC target mask, the hartid), and how many it has received. */
+    uint64_t          ipi_target;
+    volatile uint64_t ipi_received;
 } sk_percpu_t;
 
 /* Installs the boot core's block in this core's per-CPU register. Call