perf: cache syslinux and grub sections between builds

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

iso/builder/build-debug.sh

@@ -114,7 +114,9 @@ export BEE_OVERLAY_DIR="${OVERLAY_DIR}"
 # mkimage stores each section in a hash-named subdir; apks_* dirs contain downloaded packages.
 if [ -d /var/tmp/bee-iso-work ]; then
     find /var/tmp/bee-iso-work -maxdepth 1 -mindepth 1 \
-        -not -name 'apks_*' -not -name 'kernel_*' -exec rm -rf {} + 2>/dev/null || true
+        -not -name 'apks_*' -not -name 'kernel_*' \
+        -not -name 'syslinux_*' -not -name 'grub_*' \
+        -exec rm -rf {} + 2>/dev/null || true
 fi
 
 # Run from /var/tmp to avoid git repo context conflicts and to ensure enough scratch space.