Fix fast-path: treat bootloader config changes as heavy

config/bootloaders was missing from the needs_full_build heavy-file
list, so changes to GRUB theme assets (e.g. bee-logo.png RGBA→RGB fix
in 333c44f) were silently skipped by the squashfs-surgery fast-path.
The old broken PNG stayed in boot/grub/live-theme/ inside the ISO.

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

iso/builder/build.sh

@@ -867,6 +867,7 @@ needs_full_build() {
         "${BUILDER_DIR}/config/package-lists" \
         "${BUILDER_DIR}/config/hooks" \
         "${BUILDER_DIR}/config/archives" \
+        "${BUILDER_DIR}/config/bootloaders" \
         -newer "${FULL_BUILD_MARKER}" 2>/dev/null | head -1)
 
     if [ -n "$_heavy" ]; then