refactor: harden diagnostics and consolidate runtime code

iso/builder/lib/iso-validation.sh

@@ -359,9 +359,12 @@ validate_live_cmdline_params() {
             bad = 1
         }
 
-        $1 == command && has("boot=live") {
+        $1 == command {
             live_entries++
 
+            if (!has("boot=live")) {
+                reject("missing boot=live")
+            }
             if (!has("udev.children_max=1")) {
                 reject("missing udev.children_max=1")
             }
@@ -432,7 +435,7 @@ validate_iso_live_boot_entries() {
         exit 1
     }
 
-    if grep -q '@APPEND_LIVE@\|@KERNEL_LIVE@\|@INITRD_LIVE@' "$grub_cfg" "$isolinux_cfg"; then
+    if grep -Eq '@[A-Z][A-Z_]*@' "$grub_cfg" "$isolinux_cfg"; then
         echo "ERROR: unresolved live-build placeholders remain in ISO bootloader config" >&2
         rm -f "$grub_cfg" "$isolinux_cfg"
         exit 1