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
+[[ "${1:-}" =~ ^[0-9]+$ ]]

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

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+printf '%s\n' "$*" >> "$MOCK_VAGRANT_LOG"
+case "${1:-}" in
+  up)
+    counter_file="$MOCK_VBOX_STATE/counter"
+    counter=0
+    [[ ! -r "$counter_file" ]] || counter="$(cat "$counter_file")"
+    counter=$((counter + 1))
+    printf '%s\n' "$counter" > "$counter_file"
+    uuid="00000000-0000-4000-8000-$(printf '%012d' "$counter")"
+    vm_dir="$MOCK_VBOX_ROOT/master-$counter"
+    mkdir -p -- "$vm_dir"
+    : > "$vm_dir/master.vbox"
+    : > "$vm_dir/master.vdi"
+    printf '%s\n' \
+      'VMState="poweroff"' \
+      'groups="/"' \
+      "CfgFile=\"$vm_dir/master.vbox\"" \
+      "SATA-0-0=\"$vm_dir/master.vdi\"" > "$MOCK_VBOX_STATE/vm-$uuid"
+    printf '%s\n' "$uuid" > "$MOCK_BOX_DIR/master_id"
+    ;;
+  destroy) ;;
+  *) exit 2 ;;
+esac

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

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+printf '%s\n' "$*" >> "$MOCK_VBOX_LOG"
+command_name="${1:-}"
+uuid="${2:-}"
+
+case "$command_name" in
+  showvminfo)
+    [[ -r "$MOCK_VBOX_STATE/vm-$uuid" ]] || exit 1
+    cat "$MOCK_VBOX_STATE/vm-$uuid"
+    ;;
+  getextradata)
+    key_slug="${3//\//_}"
+    if [[ ! -r "$MOCK_VBOX_STATE/extra-$uuid-$key_slug" ]]; then
+      printf '%s\n' 'No value set!'
+      exit 0
+    fi
+    printf 'Value: '
+    cat "$MOCK_VBOX_STATE/extra-$uuid-$key_slug"
+    ;;
+  modifyvm)
+    [[ "${3:-}" == --groups ]]
+    awk -v groups="${4:-}" '
+      $1 !~ /^groups=/ { print }
+      END { printf "groups=\"%s\"\n", groups }
+    ' "$MOCK_VBOX_STATE/vm-$uuid" > "$MOCK_VBOX_STATE/vm-$uuid.tmp"
+    mv "$MOCK_VBOX_STATE/vm-$uuid.tmp" "$MOCK_VBOX_STATE/vm-$uuid"
+    ;;
+  setextradata)
+    key_slug="${3//\//_}"
+    printf '%s\n' "${4:-}" > "$MOCK_VBOX_STATE/extra-$uuid-$key_slug"
+    ;;
+  unregistervm|closemedium)
+    printf '%s\n' 'destructive VirtualBox command invoked' >&2
+    exit 99
+    ;;
+  *) exit 2 ;;
+esac