Add 0.5B Alpamayo validation

tests/test_diffusion.py

@@ -0,0 +1,11 @@
+import torch
+
+from alpamayo_lite.diffusion import FlowMatching
+
+
+def test_flow_matching_target_velocity() -> None:
+    flow = FlowMatching((4, 2), inference_steps=2)
+    target = torch.ones(1, 4, 2)
+    data = flow.construct_training_data(target)
+    exact_velocity = data["target_action"] - data["noise"]
+    assert float(flow.loss(data, exact_velocity)) == 0.0