test(verifier): cross-runtime goja/node parity gate on the shared picker

pkg/spec/test/parity-harness.ts

@@ -0,0 +1,37 @@
+// Cross-runtime parity harness. Run under node (tsx) by the Go test
+// internal/verifier/parity_test.go, which feeds the SAME seed and candidate
+// list into the goja verifier and asserts an identical action stream. Both
+// engines run the SAME pick.ts over the SAME Pcg, so any divergence here is an
+// engine-level (number/bigint) parity bug.
+//
+// Inputs come from env so the Go side controls them:
+//   SANDERLING_PARITY_SEED       decimal uint64 seed
+//   SANDERLING_PARITY_STEPS      action count to emit
+//   SANDERLING_PARITY_CANDIDATES JSON array of {x,y,selector,width,height}
+//
+// Output: one JSON array of serialized actions on stdout.
+
+import { installRuntime } from "../src/runtime-entry.ts";
+import type { BuiltinVerb, Candidate, GeneratorNode, Host } from "../src/action-tree.ts";
+
+const seed = BigInt(process.env.SANDERLING_PARITY_SEED ?? "0");
+const steps = Number(process.env.SANDERLING_PARITY_STEPS ?? "0");
+const candidates = JSON.parse(process.env.SANDERLING_PARITY_CANDIDATES ?? "[]") as Candidate[];
+
+const host: Host = {
+  platform: () => "android",
+  queryCandidates: (_verb: BuiltinVerb) => candidates,
+  reportUnsupported: () => {},
+  seedHi: () => seed,
+  seedLo: () => 0n,
+};
+
+const root: GeneratorNode = { kind: "builtin", verb: "taps" };
+
+installRuntime(host, root, () => ({}));
+
+const nextAction = (globalThis as { __sanderlingNextAction__: () => unknown })
+  .__sanderlingNextAction__;
+const stream = [];
+for (let i = 0; i < steps; i++) stream.push(nextAction());
+process.stdout.write(JSON.stringify(stream));