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>

config/ato.e2e.json

@@ -0,0 +1,7 @@
+{
+  "trainsim_url": "http://trainsim:8080",
+  "target_speed_mps": 8.0,
+  "tick_ms": 100,
+  "run_secs": 15,
+  "mode": "cruise"
+}