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/pmm.h

@@ -0,0 +1,36 @@
+#ifndef PMM_H
+#define PMM_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+#define PAGE_SIZE 4096
+
+typedef uintptr_t phys_addr_t;
+
+typedef enum {
+    PMM_ZONE_DMA,    // < 16 MB
+    PMM_ZONE_NORMAL, // Any other memory
+    PMM_ZONE_HIGHMEM // > 4 GB (if 64-bit or PAE, but we are 32-bit for now) - actually sticking to DMA and NORMAL is enough for 32-bit typically
+} pmm_zone_t;
+
+/*
+ * Initialize the physical memory manager.
+ * @param multiboot_addr Address of the multiboot info structure
+ */
+void init_pmm(uint32_t multiboot_addr);
+
+/*
+ * Allocate a physical page.
+ * @param zone The preferred zone to allocate from.
+ * @return Physical address of the allocated page, or 0 if out of memory.
+ */
+phys_addr_t pmm_alloc_page(pmm_zone_t zone);
+
+/*
+ * Free a physical page.
+ * @param addr Physical address of the page to free.
+ */
+void pmm_free_page(phys_addr_t addr);
+
+#endif