refactor: make pmm.c fully architecture-independent

refactor: make pmm.c fully architecture-independent

Extract all Multiboot2 x86-specific code from src/mm/pmm.c into
src/arch/x86/pmm_boot.c as pmm_arch_init().

Design:
- pmm.h now exposes pmm_mark_region(), pmm_set_limits(), pmm_arch_init()
- pmm_init() calls pmm_arch_init() (arch-specific) which discovers
  memory and calls pmm_set_limits() + pmm_mark_region()
- pmm.c provides a weak default pmm_arch_init() for archs without one
- Kernel protection uses hal_mm_virt_to_phys() (no #if MIPS/x86)
- x86 pmm_boot.c handles Multiboot2 parsing, module protection,
  and boot info protection
- Zero #if guards remain in pmm.c

Passes: make, cppcheck, QEMU smoke test.