Add standalone ATO controller in lean Rust.

Pure ato-core logic, minimal HTTP client, JSON config and spec files.
Docker build with tests; integrates with TrainSim over simple REST API.

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

spec/train_state.json

@@ -0,0 +1,6 @@
+{
+  "speed_mps": 0.0,
+  "position_m": 0.0,
+  "track_length_m": 1000.0,
+  "sim_running": false
+}