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>
