Add video_cycles_scenes to save Blender Cycles scenes of run videos

[yichao-liang]

Summary

Adds an opt-in flag that saves a Blender Cycles scene of every frame of a run's videos, to render later on a GPU node with the paper figures' materials, lighting and cameras.
The EMPIRIC project page, its X thread and the RoboDisco site now show videos rendered this way.

The flag.
--video_cycles_scenes True (default off) records a scene per frame as test and failure videos render, into <video_dir>/<video name>_cycles, so nothing that only stepping draws is lost.
A finished continual run exports its video's frames from the recorded states into <run_dir>/cycles, and scripts/continual_video.py --cycles does the same for an earlier run.
Nothing renders during a run.

Rendering.
scripts/cycles_video.py <manifest> renders the frames with Blender (bpy 4.5.3 through uv run), skipping frames already rendered so that a preempted job resumes, and assembles the videos: the run video's panel layout, the test task alone, or the scene alone for a test video.
--tasks N --task k splits the frames across jobs; on an L40S a frame takes about three seconds.

Refactor.
The scene export moves from scripts/paper_figures into predicators/run/cycles_video.py, and the paper figure scripts import it from there.
render_cycles_frames.py is the batch renderer and shares render_cycles_scene.py's scene build.
docs/RENDERING.md documents the pipeline.

README.
The Fan row still described the walled maze, the default before September 28; its preview GIF now shows the paper's ramp scene, and the row says so.

Test plan

• tests/run/test_cycles_video.py (6 tests): a run's export matches its video frame for frame, state exports and the script, Boil's heat, the run-end hook and the offline export, the live test-video monitor, and a clean exit with the recording kept.
• CI replay on 57acac9 in CI's Ubuntu 24.04 container (~/ci-tools): all 8 pytest shards pass, and yapf, isort 5.10.1, docformatter, mypy and pylint are clean.
• Rendered the EMPIRIC paper runs (frame counts match the harness videos) and all 22 RoboDisco solves; the 17 non-benchmark solves were recorded live through the flag.

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