feat: hello-world user-mode app loaded from initrd via VFS

- Created apps/hello-world/hello-world.S: AT&T assembly user program that
  calls SYS_WRITE to print 'Hello, World' then SYS_EXIT(0)
- Created apps/user.ld: linker script for user apps at 0x08048000
- Created scripts/build_apps.sh: builds each app dir into flat binary
- Updated CMakeLists.txt: added apps build target in ISO pipeline
- Updated gen_initrd.sh: packs built binaries from build/apps_bin/
- Updated kernel.c: replaced inline machine code with VFS-based loading
  of hello-world from /initrd/hello-world via cpio_find()

Verified: hello-world binary (49 bytes) loads from CPIO initrd,
prints 'Hello, World', and exits with code 0.

CMakeLists.txt

@@ -19,13 +19,20 @@ add_subdirectory(src)
 file(MAKE_DIRECTORY ${CMAKE_SOURCE_DIR}/release)
 file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/isodir/boot/grub)
 
-# Generate CPIO initial ramdisk from apps directory.
-# All files in apps/ are packed into a newc-format CPIO archive.
+# Build user-mode applications as flat binaries.
+set(APPS_BIN_DIR ${CMAKE_BINARY_DIR}/apps_bin)
+file(MAKE_DIRECTORY ${APPS_BIN_DIR})
+add_custom_target(apps
+    COMMAND ${CMAKE_SOURCE_DIR}/scripts/build_apps.sh ${CMAKE_SOURCE_DIR}/apps ${APPS_BIN_DIR}
+    COMMENT "Building user-mode applications"
+)
+
+# Generate CPIO initial ramdisk from built app binaries.
 set(INITRD_FILE ${CMAKE_BINARY_DIR}/isodir/boot/initrd.cpio)
 add_custom_command(
     OUTPUT ${INITRD_FILE}
-    COMMAND ${CMAKE_SOURCE_DIR}/scripts/gen_initrd.sh ${CMAKE_SOURCE_DIR}/apps ${INITRD_FILE}
-    DEPENDS ${CMAKE_SOURCE_DIR}/apps
+    COMMAND ${CMAKE_SOURCE_DIR}/scripts/gen_initrd.sh ${APPS_BIN_DIR} ${INITRD_FILE}
+    DEPENDS apps
     COMMENT "Generating CPIO initial ramdisk"
 )
 add_custom_target(initrd DEPENDS ${INITRD_FILE})