perf: use lz4 compression for modloop squashfs

xz → lz4 for mksquashfs: kernel modloop rebuild is ~10x faster.
Size increase is acceptable since modloop is loaded into RAM.
Applied in both setup-builder.sh and build-debug.sh.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/setup-builder.sh

@@ -122,6 +122,11 @@ if [ ! -f "${HOME}/.abuild/abuild.conf" ]; then
     fi
 fi
 
+# --- mkinitfs: use lz4 compression for faster modloop builds ---
+mkdir -p /etc/mkinitfs
+grep -q 'MKSQUASHFS_OPTS' /etc/mkinitfs/mkinitfs.conf 2>/dev/null || \
+    echo 'MKSQUASHFS_OPTS="-comp lz4 -Xhc"' >> /etc/mkinitfs/mkinitfs.conf
+
 echo ""
 echo "=== builder setup complete ==="
 echo "Next: sh iso/builder/build-debug.sh"