Use dataclasses.replace in _finalize_summary

[davidkfoss]

Problem

_finalize_summary (episode.py:374-396) re-lists every field of EpisodeSummary by hand just to attach 5 reward-related fields. Any new field added to EpisodeSummary must also be added here, or it gets silently dropped on round-trip.

Proposal

Use dataclasses.replace:

return dataclasses.replace(
    summary,
    reward=reward,
    reward_base=reward_base,
    ...,
)

Removes the manual field list and the implicit "add it in two places" coupling.

Acceptance criteria

• _finalize_summary uses dataclasses.replace.
• No behavior change; existing tests pass.