test(spec): golden-fixture cross-runtime parity gate for the node picker

Replace the env-driven parity harness with a shared scenario module and a
committed golden the node picker asserts independently. The goja side asserts
the same golden, so neither runtime invokes the other at test time.

pkg/spec/test/parity.test.ts

@@ -0,0 +1,24 @@
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { test } from "node:test";
+
+import { PARITY_STEPS, runParity } from "./parity-harness.ts";
+
+const goldenPath = fileURLToPath(new URL("./fixtures/parity-golden.json", import.meta.url));
+const golden = JSON.parse(readFileSync(goldenPath, "utf8"));
+
+// The node side of the W2 acceptance gate: the shared picker's stream for the
+// fixed parity scenario must match the committed golden byte-for-byte. The goja
+// test asserts the SAME golden, so a match on both sides proves cross-runtime
+// parity. A drift here means pick.ts, the corpus, or the Pcg changed.
+test("node picker matches the cross-runtime golden", () => {
+  const stream = runParity();
+  assert.equal(stream.length, PARITY_STEPS);
+  assert.deepEqual(stream, golden);
+});
+
+// Determinism: a second run from the same seed yields the identical stream.
+test("node picker is deterministic across repeated runs", () => {
+  assert.deepEqual(runParity(), runParity());
+});