GridWorld Run Artifacts (Umbrella)

Stable import for small GridWorld JSON artifacts used by TorchLean RL workflows and widgets. The implementation is split into:

• Position: shared (row, col) JSON helpers;
• Policy: before/after greedy policy snapshots;
• Path: before/after rollout trajectory snapshots.

Primitive Nat/String array codecs come from Runtime.Training.JsonCodec, so training logs and RL artifacts share one JSON vocabulary.