Add support for rendering during training in benchmark configurations

[Victorbares]

What

• Add eval.keep_zlib_replays to control whether intermediate .replay.zlib files are retained after HTML rendering.
• Delete compressed replay files by default once self-contained HTML replays are generated.
• Rename the replay directory from replays/ to replays_zlib/.
• Update evaluation documentation, configuration, and tests.

Why

Rendered HTML replays are self-contained, so retaining their compressed source files unnecessarily consumes storage.

Notes

• Set eval.keep_zlib_replays=true to retain compressed replay bundles.
• Rendered HTML pages and their index.html remain available regardless of this setting.
• Existing workflows expecting replay files under replays/ must use replays_zlib/ instead.