feat(ci): optimize Molecule VM creation

- reuse validated runner-owned Vagrant linked-clone masters\n- create the default guests in bounded two-machine batches\n- capture create timing and runner utilization diagnostics

.github/scripts/test-fixtures/mock-flock

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+[[ "${1:-}" =~ ^[0-9]+$ ]]