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.

docs/cpio.md

@@ -0,0 +1,81 @@
+# CPIO Initial Ramdisk
+
+## Overview
+
+The initial ramdisk (initrd) provides files to the kernel at boot time before
+any filesystem drivers are available. It is a CPIO archive in SVR4/newc format,
+loaded by GRUB as a Multiboot2 module.
+
+## Build Process
+
+During the build, the script `scripts/gen_initrd.sh` packs all files from the
+`apps/` directory into a CPIO archive:
+
+```
+apps/
+├── README          (placeholder)
+├── hello-world     (future: flat binary)
+└── sh              (future: shell binary)
+```
+
+The archive is placed at `build/isodir/boot/initrd.cpio` and included in the
+ISO image. GRUB loads it as a module via:
+
+```
+module2 /boot/initrd.cpio
+```
+
+## CPIO Format
+
+The newc (SVR4) CPIO format uses 110-byte headers with hex ASCII fields:
+
+```
+Offset  Size  Field
+0       6     Magic ("070701")
+6       8     Inode
+14      8     Mode
+22      8     UID
+30      8     GID
+38      8     Nlink
+46      8     Mtime
+54      8     Filesize
+62      8     Devmajor
+70      8     Devminor
+78      8     Rdevmajor
+86      8     Rdevminor
+94      8     Namesize
+102     8     Check
+```
+
+After the header: filename (namesize bytes, padded to 4-byte boundary),
+then file data (filesize bytes, padded to 4-byte boundary). The archive
+ends with a `TRAILER!!!` entry.
+
+## Kernel Interface
+
+The kernel finds the initrd by scanning Multiboot2 boot information for
+`MULTIBOOT_TAG_TYPE_MODULE` (type 3). The module's physical memory range
+is identity-mapped, so it can be read directly.
+
+```c
+#include "cpio.h"
+
+// Find a file
+cpio_entry_t entry;
+if (cpio_find("hello-world", &entry) == 0) {
+    // entry.data = pointer to file contents
+    // entry.datasize = file size
+}
+
+// Iterate all files
+uint32_t offset = 0;
+while (cpio_next(&offset, &entry) == 0) {
+    // entry.name, entry.data, entry.datasize
+}
+```
+
+## Files
+
+- `scripts/gen_initrd.sh` — Build script to generate CPIO archive
+- `src/cpio.h` / `src/cpio.c` — CPIO parser (find, iterate, count)
+- `CMakeLists.txt` — `initrd` target and `module2` in grub.cfg