test: add FuzzMarshal, golden messages, synctest cancellation and cross smoke

Assisted-by: GLM 5.3 Flash

interpres_test.go

@@ -796,3 +796,28 @@ func TestParseAsAndNewSchema(t *testing.T) {
 	}
 	NewSchema[map[string]any]() // must not panic
 }
+
+func TestZeroOffsetRoundTrip(t *testing.T) {
+	// A document may write a zero offset as +00:00; the tree must hold the
+	// same value after a round trip, because the written form is "Z" either
+	// way.
+	src := []byte("a = 1979-05-27T07:32:00+00:00\n")
+	tree, err := ParseMap(src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out, err := Marshal(tree)
+	if err != nil {
+		t.Fatal(err)
+	}
+	re, err := ParseMap(out)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(tree, re) {
+		t.Errorf("round trip changed the tree: %#v vs %#v", tree, re)
+	}
+	if got := tree["a"].(OffsetDateTime).String(); got != "1979-05-27T07:32Z" {
+		t.Errorf("a = %q, want 1979-05-27T07:32Z", got)
+	}
+}