interrupts: get_apic_base() uses hard-coded 0xFEE00000 (magic number)

[emilf]

kernel/src/interrupts/apic.rs:get_apic_base() currently returns a constant 0xFEE00000 instead of reading IA32_APIC_BASE (MSR 0x1B) or otherwise deriving the LAPIC base.\n\nThis is brittle on systems with relocated LAPIC and is also a barrier to a clean x2APIC transition (where register access should be MSR-based).\n\nAcceptance:\n- read IA32_APIC_BASE MSR when safe (early boot constraints noted)\n- plumb the physical base through mapping code (or document invariants)\n- eliminate the magic constant usage