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>

spec/interface.md

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+# ATO ↔ TrainSim interface (v1)
+
+Standalone. No ETCS. No ATP. JSON over HTTP.
+
+Base URL (TrainSim): `http://trainsim:8080`
+
+## TrainSim → ATO reads
+
+`GET /train/state`
+
+```json
+{
+  "speed_mps": 0.0,
+  "position_m": 0.0,
+  "track_length_m": 1000.0,
+  "sim_running": false
+}
+```
+
+## ATO → TrainSim writes
+
+`POST /train/actuate`
+
+```json
+{
+  "traction_pct": 0,
+  "brake_pct": 0
+}
+```
+
+Percent 0–100. Do not send both > 0 (TrainSim clamps brake wins).
+
+`POST /sim/start` — start physics tick (empty body)
+
+`POST /sim/stop` — pause physics (empty body)
+
+Optional (TrainSim logs only): `POST /train/doors` `{ "command": "open" | "close" }`
+
+## ATO configuration (env)
+
+| Variable | Meaning |
+|----------|---------|
+| `TRAINSIM_URL` | TrainSim base URL |
+| `TARGET_SPEED_MPS` | Cruise target (default 10) |
+| `TICK_MS` | Control period (default 100) |