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_physics.py

@@ -0,0 +1,22 @@
+from trainsim.config_loader import SimParams
+from trainsim.physics import Train
+
+
+def test_train_accelerates_with_traction():
+    p = SimParams(1000, 20, 0.15, 0.25, 0.02, 25)
+    t = Train()
+    t.sim_running = True
+    t.actuate(100, 0)
+    for _ in range(120):
+        t.tick(p, 0.05)
+    assert t.speed_mps > 0.5
+
+
+def test_brake_slows_train():
+    p = SimParams(1000, 20, 0.15, 0.25, 0.02, 25)
+    t = Train(speed_mps=10.0)
+    t.sim_running = True
+    t.actuate(0, 100)
+    for _ in range(200):
+        t.tick(p, 0.05)
+    assert t.speed_mps < 10.0