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/cpio.h

@@ -0,0 +1,92 @@
+/**
+ * @file cpio.h
+ * @brief CPIO newc archive parser.
+ *
+ * Parses CPIO archives in the SVR4/newc format (magic "070701").
+ * Used to read files from the initial ramdisk loaded by GRUB.
+ */
+
+#ifndef CPIO_H
+#define CPIO_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+/**
+ * CPIO newc header (110 bytes).
+ * All fields are 8-character hexadecimal ASCII strings.
+ */
+typedef struct cpio_newc_header {
+    char magic[6];       /**< Must be "070701". */
+    char ino[8];
+    char mode[8];
+    char uid[8];
+    char gid[8];
+    char nlink[8];
+    char mtime[8];
+    char filesize[8];
+    char devmajor[8];
+    char devminor[8];
+    char rdevmajor[8];
+    char rdevminor[8];
+    char namesize[8];
+    char check[8];
+} cpio_newc_header_t;
+
+/** Size of the CPIO newc header in bytes. */
+#define CPIO_HEADER_SIZE 110
+
+/** CPIO newc magic string. */
+#define CPIO_MAGIC "070701"
+
+/** Trailer entry name that marks end of archive. */
+#define CPIO_TRAILER "TRAILER!!!"
+
+/**
+ * CPIO file entry (result of iteration or lookup).
+ */
+typedef struct cpio_entry {
+    const char *name;    /**< Filename (pointer into archive). */
+    uint32_t namesize;   /**< Length of filename including NUL. */
+    const void *data;    /**< Pointer to file data within archive. */
+    uint32_t datasize;   /**< Size of file data in bytes. */
+    uint32_t mode;       /**< File mode/permissions. */
+} cpio_entry_t;
+
+/**
+ * Initialize the CPIO parser with the archive location.
+ *
+ * @param archive_start Pointer to the start of the CPIO archive in memory.
+ * @param archive_size  Size of the archive in bytes (0 if unknown).
+ */
+void cpio_init(const void *archive_start, uint32_t archive_size);
+
+/**
+ * Find a file in the CPIO archive by name.
+ *
+ * @param name    Filename to search for (without leading "./").
+ * @param entry   Output: filled with file information if found.
+ * @return 0 on success, -1 if not found.
+ */
+int cpio_find(const char *name, cpio_entry_t *entry);
+
+/**
+ * Iterate through all entries in the CPIO archive.
+ *
+ * Call with *offset = 0 to start. Returns 0 on success, -1 when no
+ * more entries exist or the TRAILER is reached.
+ *
+ * @param offset  In/out: current position in the archive.
+ * @param entry   Output: filled with the next entry's information.
+ * @return 0 on success, -1 at end of archive.
+ */
+int cpio_next(uint32_t *offset, cpio_entry_t *entry);
+
+/**
+ * Get the number of files in the CPIO archive (excluding TRAILER).
+ *
+ * @return Number of files.
+ */
+uint32_t cpio_count(void);
+
+#endif /* CPIO_H */