Add Python TrainSim with loop track map and physics.

FastAPI server, replaceable JSON config, tests, Dockerfile.
Pairs with ATO via spec/interface.md contract.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_api.py

@@ -0,0 +1,26 @@
+import os
+
+from fastapi.testclient import TestClient
+
+os.environ.setdefault("SIM_CONFIG", "config/sim.default.json")
+os.environ.setdefault("TRACK_CONFIG", "config/track.default.json")
+
+from trainsim.app import create_app  # noqa: E402
+
+client = TestClient(create_app())
+
+
+def test_health():
+    assert client.get("/health").json()["status"] == "ok"
+
+
+def test_state_shape():
+    s = client.get("/train/state").json()
+    assert "speed_mps" in s and "track_length_m" in s
+
+
+def test_actuate_and_start():
+    client.post("/sim/start")
+    client.post("/train/actuate", json={"traction_pct": 50, "brake_pct": 0})
+    s = client.get("/train/state").json()
+    assert s["sim_running"] is True