feat(spec): reject an unknown object-selector key in the web runtime

Same rule and the same message as the native side: a key no element can
carry throws instead of matching nothing. The accepted list is one list,
committed as a fixture both suites assert, so a spec cannot be accepted
by one runtime and rejected by the other.

pkg/spec/test/selector-keys.test.ts

@@ -0,0 +1,21 @@
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { test } from "node:test";
+
+import { __testing__ } from "../src/web-runtime.ts";
+
+const { SELECTOR_KEYS } = __testing__;
+
+const goldenPath = fileURLToPath(new URL("./fixtures/selector-keys.json", import.meta.url));
+const golden: string[] = JSON.parse(readFileSync(goldenPath, "utf8"));
+
+// Both runtimes reject an object-selector key they do not know, so the two key
+// lists have to be one list. Were they to drift, a spec would be accepted by
+// the runtime that lists the key and fail the run on the one that does not, and
+// the difference would only show on the platform nobody ran first.
+// internal/hierarchy/selector_keys_test.go asserts the SAME file from the
+// native side.
+test("the web key list is the cross-runtime list", () => {
+  assert.deepEqual([...SELECTOR_KEYS], golden);
+});