Add CPIO initial ramdisk with build infrastructure and parser (AI)

Build system changes:
- scripts/gen_initrd.sh packs all files from apps/ into a newc-format
  CPIO archive at build/isodir/boot/initrd.cpio.
- CMakeLists.txt adds 'initrd' target as ISO dependency. GRUB loads the
  archive as a Multiboot2 module via 'module2 /boot/initrd.cpio'.
- apps/README added as placeholder file for initial ramdisk content.

Kernel changes:
- kernel.c scans Multiboot2 tags for MULTIBOOT_TAG_TYPE_MODULE to find
  the initrd's physical address range, then passes it to cpio_init().
- cpio.c/h implements a parser for the SVR4/newc CPIO format:
  - cpio_init(): lists archive contents on startup
  - cpio_find(): look up a file by name (handles ./ prefix)
  - cpio_next(): iterate through all entries
  - cpio_count(): count files in archive
- The initrd lives in identity-mapped physical memory, so no additional
  mapping is needed to access it.

Verified in QEMU: GRUB loads the module at 0x0014A000, CPIO parser
finds the README file (38 bytes). All existing functionality (Ring 3
processes, syscalls) continues to work.

src/kernel.c

@@ -13,6 +13,7 @@
 #include "tss.h"
 #include "syscall.h"
 #include "process.h"
+#include "cpio.h"
 
 void offset_print(const char *str)
 {
@@ -54,6 +55,26 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     init_pmm(addr);
     offset_print("PMM initialized\n");
 
+    /* Scan Multiboot2 tags for the initrd module */
+    uint32_t initrd_start = 0, initrd_end = 0;
+    {
+        struct multiboot_tag *tag;
+        for (tag = (struct multiboot_tag *)(addr + 8);
+             tag->type != MULTIBOOT_TAG_TYPE_END;
+             tag = (struct multiboot_tag *)((uint8_t *)tag + ((tag->size + 7) & ~7u))) {
+            if (tag->type == MULTIBOOT_TAG_TYPE_MODULE) {
+                struct multiboot_tag_module *mod = (struct multiboot_tag_module *)tag;
+                initrd_start = mod->mod_start;
+                initrd_end = mod->mod_end;
+                offset_print("Initrd module at ");
+                print_hex(initrd_start);
+                offset_print(" to ");
+                print_hex(initrd_end);
+                break; /* Use first module */
+            }
+        }
+    }
+
     init_paging();
     offset_print("Paging initialized\n");
 
@@ -72,6 +93,14 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     init_kmalloc();
     offset_print("Memory allocator initialized\n");
 
+    /* Initialize CPIO ramdisk if module was loaded */
+    if (initrd_start != 0) {
+        cpio_init((const void *)initrd_start, initrd_end - initrd_start);
+        offset_print("CPIO ramdisk initialized\n");
+    } else {
+        offset_print("No initrd module found\n");
+    }
+
     init_tss();
     offset_print("TSS initialized\n");