Implement physical memory allocator with zone support (AI)

- Added bitmap-based physical memory manager (PMM) that parses the Multiboot2
  memory map to discover available RAM regions.
- Supports two allocation zones: PMM_ZONE_DMA (below 16MB) and PMM_ZONE_NORMAL
  (above 16MB), with automatic fallback from NORMAL to DMA when the preferred
  zone is exhausted.
- Marks kernel memory region and multiboot info structure as reserved using
  _kernel_start/_kernel_end linker symbols.
- Page 0 is always marked as used to prevent returning NULL as a valid address.
- Added linker script symbols (_kernel_start, _kernel_end) to track kernel
  memory boundaries for the allocator.
- Kernel now initializes PMM after PIC and performs test allocations to verify
  the subsystem works.

src/linker.ld

@@ -4,6 +4,8 @@ SECTIONS
 {
 	. = 1M;
 
+    _kernel_start = .;
+
 	.text BLOCK(4K) : ALIGN(4K)
 	{
 		KEEP(*(.multiboot))
@@ -25,4 +27,6 @@ SECTIONS
 		*(COMMON)
 		*(.bss)
 	}
+
+    _kernel_end = .;
 }