perf(iso): speed up builds — zstd squashfs + preserve lb chroot cache

- Switch squashfs compression from xz to zstd (3-5x faster compression,
  ~10-15% larger but decompresses faster at boot)
- Stop rm -rf BUILD_WORK_DIR on each build; rsync only config changes
  so lb can reuse its chroot across builds (skips apt install step)
- Keep lb-packages cache in CACHE_ROOT as fallback if work dir is wiped

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

iso/builder/build.sh

@@ -200,17 +200,24 @@ else
 fi
 
 echo "=== preparing staged overlay ==="
-# Preserve lb package cache across builds so apt packages are not re-downloaded.
+# Sync builder config into work dir, preserving lb cache (chroot + packages).
+# We do NOT rm -rf BUILD_WORK_DIR so lb can reuse its chroot on repeat builds.
+mkdir -p "${BUILD_WORK_DIR}" "${OVERLAY_STAGE_DIR}"
+rsync -a --delete \
+    --exclude='cache/' \
+    --exclude='chroot/' \
+    --exclude='.build/' \
+    --exclude='*.iso' \
+    --exclude='*.packages' \
+    --exclude='*.contents' \
+    --exclude='*.files' \
+    "${BUILDER_DIR}/" "${BUILD_WORK_DIR}/"
+# Also persist package cache to CACHE_ROOT so it survives a manual wipe of BUILD_WORK_DIR.
 LB_PKG_CACHE="${CACHE_ROOT}/lb-packages"
 mkdir -p "${LB_PKG_CACHE}"
 if [ -d "${BUILD_WORK_DIR}/cache/packages.chroot" ]; then
     rsync -a --delete "${BUILD_WORK_DIR}/cache/packages.chroot/" "${LB_PKG_CACHE}/"
-fi
-rm -rf "${BUILD_WORK_DIR}" "${OVERLAY_STAGE_DIR}"
-mkdir -p "${BUILD_WORK_DIR}" "${OVERLAY_STAGE_DIR}"
-rsync -a "${BUILDER_DIR}/" "${BUILD_WORK_DIR}/"
-# Restore lb package cache
-if [ -d "${LB_PKG_CACHE}" ] && [ "$(ls -A "${LB_PKG_CACHE}" 2>/dev/null)" ]; then
+elif [ -d "${LB_PKG_CACHE}" ] && [ "$(ls -A "${LB_PKG_CACHE}" 2>/dev/null)" ]; then
     mkdir -p "${BUILD_WORK_DIR}/cache/packages.chroot"
     rsync -a "${LB_PKG_CACHE}/" "${BUILD_WORK_DIR}/cache/packages.chroot/"
 fi