Remove unused _loss_delta_ema; use EMATracker for one-shot EMA in _build_episode_summary

[davidkfoss]

Problems

1. _loss_delta_ema is updated (episode.py:143, :410) but never read anywhere.
2. _build_episode_summary (episode.py:354) re-implements a one-shot EMA pass inline instead of reusing the existing EMATracker.

Action

• Delete _loss_delta_ema and the lines that update it.
• Replace the inline EMA loop in _build_episode_summary with EMATracker.

Acceptance criteria

• _loss_delta_ema and its updates removed.
• _build_episode_summary uses EMATracker.
• Tests still pass.