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/kernel.c

@@ -5,6 +5,7 @@
 #include "idt.h"
 #include "pic.h"
 #include "port_io.h"
+#include "pmm.h"
 
 void offset_print(const char *str)
 {
@@ -46,6 +47,17 @@ void kernel_main(uint32_t magic, uint32_t addr) {
 
     init_pic();
     offset_print("PIC initialized\n");
+    
+    init_pmm(addr);
+    offset_print("PMM initialized\n");
+
+    phys_addr_t p1 = pmm_alloc_page(PMM_ZONE_NORMAL);
+    offset_print("Allocated page at: ");
+    print_hex(p1);
+    
+    phys_addr_t p2 = pmm_alloc_page(PMM_ZONE_DMA);
+    offset_print("Allocated DMA page at: ");
+    print_hex(p2);
 
     /* Enable interrupts */
     asm volatile("sti");