add pipeline_parallel_stagger function

multipass/run_parallel_iiab_test.sh

@@ -491,6 +491,28 @@ phase_parallel_stagger() {
   set -e
 }
 
+pipeline_parallel_stagger() {
+  # Per-VM pipeline: each VM does launch->install->resume independently.
+  # This avoids "install barrier" where slow Debian blocks Ubuntu's resume.
+  local pids=()
+
+  for i in "${!names[@]}"; do
+    local vm="${names[$i]}"
+    (
+      sleep $((i * STAGGER))
+      launch_one "$vm"
+      run_install_txt "$vm"
+      resume_iiab "$vm" 1
+    ) &
+    pids+=("$!")
+  done
+
+  # Don't abort the whole script if one VM fails; we still want logs + summary.
+  set +e
+  wait_all "${pids[@]}"
+  set -e
+}
+
 # ---- Main ----
 build_vm_lists