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.

scripts/build_apps.sh

@@ -0,0 +1,49 @@
+#!/bin/sh
+# Build all user-mode applications as flat binaries.
+# Usage: build_apps.sh <apps_dir> <output_dir>
+# Each app directory in <apps_dir>/ gets compiled and its flat binary
+# is placed in <output_dir>/.
+set -e
+
+APPS_DIR="$1"
+OUTPUT_DIR="$2"
+LINKER_SCRIPT="$APPS_DIR/user.ld"
+
+CC="${CC:-clang}"
+OBJCOPY="${OBJCOPY:-objcopy}"
+CFLAGS="-ffreestanding -m32 -fno-pie -fno-pic -fno-builtin -fno-stack-protector -mno-sse -mno-mmx -O2 -Wall"
+LDFLAGS="-m32 -nostdlib -no-pie -Wl,--no-dynamic-linker"
+
+mkdir -p "$OUTPUT_DIR"
+
+for app_dir in "$APPS_DIR"/*/; do
+    [ -d "$app_dir" ] || continue
+    app_name=$(basename "$app_dir")
+
+    echo "Building app: $app_name"
+
+    # Collect source files
+    OBJ_FILES=""
+    for src in "$app_dir"*.S "$app_dir"*.c; do
+        [ -f "$src" ] || continue
+        obj="$OUTPUT_DIR/${app_name}_$(basename "${src%.*}").o"
+        $CC $CFLAGS -c "$src" -o "$obj"
+        OBJ_FILES="$OBJ_FILES $obj"
+    done
+
+    if [ -z "$OBJ_FILES" ]; then
+        echo "  No sources found, skipping"
+        continue
+    fi
+
+    # Link into ELF
+    elf="$OUTPUT_DIR/$app_name.elf"
+    $CC $LDFLAGS -T "$LINKER_SCRIPT" $OBJ_FILES -o "$elf"
+
+    # Convert to flat binary (strip non-code sections)
+    bin="$OUTPUT_DIR/$app_name"
+    $OBJCOPY -O binary --only-section=.text --only-section=.rodata --only-section=.data "$elf" "$bin"
+
+    size=$(wc -c < "$bin")
+    echo "  Built: $bin ($size bytes)"
+done