[M2] Scope note: defer full APIC init

[mitsuakki]

M2 stays on legacy PIC only.

Full Local APIC/IOAPIC needs CPUID feature detection plus ACPI MADT parsing (which first needs minimal RSDP/RSDT walking), which reads as its own chunk of work rather than an add-on to interrupt plumbing. Documenting this as a deliberate scope cut, likely belongs in a later SMP-oriented milestone.

APIC and ACPI pages for reference when that milestone comes up.