Make the paper's Fan and Balloons scenes the defaults

[yichao-liang]

Summary

• Fan now defaults to the scene of the paper and scripts/configs/empiric: the exposed deck with the downhill ramp, inertial dynamics, no walls and a 3 x 3 grid, which also brings the wide camera. HISTORICAL_ARENA_FLAGS in pybullet_fan.py restores the historical uniform/maze arena, and the tests written for that arena now set it explicitly.
• Balloons draws its ceiling as an opaque red cap across the chute, the picture the paper figures use, in place of a translucent plate over the whole table. The ceiling is visual only, so no physics changes. The hatch scene keeps the plate.
• scripts/paper_figures/render_run_videos.py renders a run's labelled video from its recorded states, moved into the current scene layouts as the figures do. Replaying the actions of runs recorded before the layouts changed does not reproduce them in the current env. The project page uses it for the Balloons and Fan videos.
• The figure exporter's _load_run_config now drops launch flags that the current code no longer defines; restoring older runs failed on them since the agent flags were removed.
• docs/RENDERING.md documents both.

The committed paper-figure scenes are unchanged: re-exporting the Balloons scenes gives the same ceiling cap.

Test plan

• CI on this commit: all 14 checks green (run 36432013362).
• Every Fan and Balloons test file, per file, on this branch and on a clean master: master all pass; the five Fan files that assumed the old arena now pass with HISTORICAL_ARENA_FLAGS (80 passed, the same 4 xfails as master).
• yapf 0.32.0, isort 5.10.1, docformatter 1.4, mypy 1.8.0 and pylint on the changed files, locally.
• Rendered the Balloons and Fan paper runs: same frame counts as the originals (556 and 1650).

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