v0.6.0: ACPI static-table parsing

Section 7 ACPI lands: kernel locates the RSDP via the IA-PC scan
(EBDA + BIOS area), walks the RSDT (ACPI 1.0) or XSDT (2.0+), and
parses the two tables we actually need today: FADT for soft-off
shutdown and MADT for CPU + IOAPIC topology.

sys_shutdown now tries real ACPI S5 first (FADT-described PM1a_CNT_BLK
port write) and only falls back to the legacy QEMU / Bochs / VirtualBox
port-magic paths when ACPI is absent. MADT reports CPU count + LAPIC
base + IOAPIC base on the boot serial line, ready for the future SMP
bring-up.

No AML interpreter: SLP_TYPa is hardcoded to 5 (the value used by
essentially every PC firmware including QEMU). See TODO.md "AML
interpreter -- FUTURE" for the honest impact assessment on laptop
hardware and the path forward if we ever need it.

4 atomic commits plus a follow-up smoke-acpi test target on master.