Storage V2: lazy ExperimentResult.episodes + iterator-based bulk methods

[recursix]

Problem

ExperimentResult.episodes() reads all episode.metadata.json files eagerly on first access. load_all_trajectories() and load_all_trajectory_metadata() return list[Trajectory], loading everything into memory.

At 100k episodes, this is a significant memory and latency bottleneck.

Proposed Fix

• Make ExperimentResult.episodes() yield EpisodeResult wrappers without reading metadata — the wrapper already loads metadata lazily on first .metadata() call.
• Return iterators instead of lists from load_all_trajectories() and load_all_trajectory_metadata().
• Consider ThreadPoolExecutor for parallel metadata reads when the full list is needed.

Context

Identified during review of PR #262 (Storage V2).