Add rollout snapshot rendering to eval

[sgreenbury]

Summary

• Adds optional rollout snapshot saving during eval (still panels rendered directly from tensors).
• Adds plot_spatiotemporal_snapshots utility and wires it into rollout rendering.
• Documents and exposes config flags for snapshot dir/timesteps/channels/format.

Test plan

• Run eval with eval.save_rollout_snapshots=true and verify snapshot panels appear under videos/snapshots/.
• Confirm existing rollout MP4/GIF rendering is unchanged when snapshots are disabled.