feat: gate route_tuple helper

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_gate.py

@@ -0,0 +1,11 @@
+# tests/test_gate.py
+from gates import gate
+
+def test_route_tuple_places_image_at_chosen():
+    B = object()
+    t = gate.route_tuple(2, "IMG", B, max_routes=5)
+    assert t == (B, B, "IMG", B, B)
+
+def test_route_tuple_length_is_max():
+    B = object()
+    assert len(gate.route_tuple(0, "IMG", B, max_routes=10)) == 10