Fix PMM: switch to Multiboot2 boot protocol and add documentation (AI)

- Changed grub.cfg from 'multiboot' to 'multiboot2' command. The PMM parses
  Multiboot2 tag structures, but GRUB was booting with Multiboot1 protocol,
  causing the memory map parsing to silently fail (all memory stayed marked
  as used, leading to OOM on every allocation).
- Fixed BITMAP_SIZE calculation to properly round up instead of truncating,
  ensuring the last few pages of the address space are covered.
- Fixed sign comparison warning in bitmap init loop.
- Added debug output to PMM init (mem_upper, region count) for diagnostics.
- Removed stale Multiboot1 magic constant and (void)addr cast from kernel.c.
- Added documentation for the interrupt subsystem and PMM in docs/.
- Checked off 'Implement a PIC handler' and 'Create a physical memory
  allocator' in the task list.

Tested: kernel boots in QEMU with both 4MB and 128MB RAM, PMM correctly
allocates from NORMAL zone (0x01000000) and DMA zone (0x00001000).

src/kernel.c

@@ -15,8 +15,6 @@ void offset_print(const char *str)
     }
 }
 
-#define MULTIBOOT_BOOTLOADER_MAGIC 0x2BADB002
-
 void print_hex(uint32_t val)
 {
     const char *hex = "0123456789ABCDEF";
@@ -29,9 +27,7 @@ void print_hex(uint32_t val)
 }
 
 void kernel_main(uint32_t magic, uint32_t addr) {
-    (void)addr; // Unused for now
-
-    if (magic != MULTIBOOT2_BOOTLOADER_MAGIC && magic != MULTIBOOT_BOOTLOADER_MAGIC) {
+    if (magic != MULTIBOOT2_BOOTLOADER_MAGIC) {
         offset_print("Invalid magic number: ");
         print_hex(magic);
         return;