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/actuate.json

@@ -0,0 +1,4 @@
+{
+  "traction_pct": 0,
+  "brake_pct": 0
+}