P0: pin project runtime and document FreeCAD parity plan

scripts/check-runtime.mjs

@@ -0,0 +1,9 @@
+const expected = process.env.BITBYBIT_NODE_VERSION ?? "22.23.2";
+const actual = process.versions.node;
+
+if (actual !== expected) {
+  console.error(`Project runtime mismatch: expected Node ${expected}, got ${actual}. Use ./npmw or ./nodew.`);
+  process.exit(1);
+}
+
+console.log(`Project runtime OK: Node ${actual}`);