Add post_pos as a model target via travel_dir decomposition

step_length already encodes |post_pos - pre_pos| by definition, so a raw
post_pos target would duplicate that magnitude and could drift inconsistent
with step_length during sampling. Instead add travel_dir, a unit vector
(local frame) giving only the direction of pre_pos->post_pos; post_pos is
reconstructed at inference as pre_pos + step_length * travel_dir, keeping
the two self-consistent. Target grows from 6D to 9D.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

giant/validate.py

@@ -11,6 +11,9 @@ _TARGET_NAMES = [
     "post_dx",
     "post_dy",
     "post_dz",
+    "travel_dx",
+    "travel_dy",
+    "travel_dz",
 ]
 
 
@@ -24,7 +27,7 @@ def validate_marginals(
 ) -> dict[str, np.ndarray]:
     """Compare per-dimension marginals of generated vs. real steps.
 
-    Returns {"real": (N,6), "generated": (N,6)} in normalised space.
+    Returns {"real": (N,9), "generated": (N,9)} in normalised space.
     """
     if device is None:
         device = next(model.parameters()).device