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.

scripts/gen_initrd.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+# Generate CPIO initial ramdisk from apps directory
+# Usage: gen_initrd.sh <apps_dir> <output_file>
+set -e
+
+APPS_DIR="$1"
+OUTPUT="$2"
+
+# Ensure output directory exists
+mkdir -p "$(dirname "$OUTPUT")"
+
+cd "$APPS_DIR"
+find . -not -name '.' | cpio -o -H newc > "$OUTPUT" 2>/dev/null
+echo "Generated initrd: $(wc -c < "$OUTPUT") bytes"