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.

internal/hierarchy/selector_keys_test.go

@@ -0,0 +1,39 @@
+package hierarchy
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"slices"
+	"testing"
+)
+
+// 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.
+// pkg/spec/test/selector-keys.test.ts asserts the SAME file from the web side.
+func TestSelectorKeysMatchTheCrossRuntimeList(t *testing.T) {
+	path, err := filepath.Abs("../../pkg/spec/test/fixtures/selector-keys.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	body, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read selector keys: %v", err)
+	}
+	var want []string
+	if err := json.Unmarshal(body, &want); err != nil {
+		t.Fatalf("decode selector keys: %v", err)
+	}
+	if got := SelectorKeys(); !slices.Equal(got, want) {
+		t.Errorf("native key list\n got=%v\nwant=%v", got, want)
+	}
+}
+
+func TestSelectorKeysAreSorted(t *testing.T) {
+	keys := SelectorKeys()
+	if !slices.IsSorted(keys) {
+		t.Errorf("keys must stay sorted so the two lists compare readably: %v", keys)
+	}
+}