Declare Node 12 as the floor.

The engines field said node >= 4 since 2018. This package runs on
Amber, which has required an ES2020 engine - Node 12 or newer - since
0.30.0 and states so from 0.31.0; nothing older can load it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

package.json

@@ -22,7 +22,7 @@
     }
   ],
   "engines": {
-    "node": ">=4.0.0"
+    "node": ">=12.0.0"
   },
   "scripts": {
     "reset": "npm run clean && npm run init",