feat(simpler_env): real-policy success videos + fix drawer/place/widowx upstream fidelity

The doc gallery was a scripted parity demo (one generic reach-close-lift motion for
all 25 tasks) — it never actually solved anything. Replace it with REAL pretrained
policies driving our MetaSim-native env, and fix genuine fidelity bugs found in the
process.

Real-policy gallery (scripts/render_policy_gallery.py):
- RT-1 (rt_1_tf_trained_for_000400120) for the 21 Google-robot tasks, Octo-base for
  the 4 WidowX tasks — matching what SimplerEnv evaluates per embodiment. Records the
  first episode the task's own success checker marks solved.
- 13/25 solved on our MetaSim-native env (5 picks + 3 move-near + 5 drawers). place
  (0/5) and widowx (0/4) are genuine policy limits — verified identical on upstream
  simpler_env (RT-1 place ~0, Octo bridge ~0 there too), not an integration defect.

Fidelity bugs fixed (the "fake-parity trap": our native-vs-_native parity was Δ=0 but
both sides shared the same deviation from upstream, so it hid these):
- drawer + place never implemented SimplerEnv's visual-matching STATION system
  (per-episode robot base xy+yaw from the fixed table + the matching real-image
  overlay open_drawer_<a0..c2>.png); they used a coke-style random base and no overlay.
- the cabinet used mk_station.urdf (white) instead of mk_station_recolor.urdf (tan,
  matches the overlay) — upstream station_name is "mk_station_recolor".
- widowx put-on never applied the bridge overlay (bridge_real_eval_1 / bridge_sink).
Result: initial obs now matches upstream by mean-abs <= ~2/255 across all 6 families
(coke/pick/move-near bitwise; drawer 0.01, place 1.94, widowx 0.0). This upstream-obs
check supersedes the native-vs-reference parity in the Status table.

_assets.py: add cabinet_recolor_urdf (vendored + on HF). Doc gallery regenerated
data-driven from the rollout results (scripts/gen_simpler_doc_gallery.py): 25 videos
with honest per-task ✓ steps / ✗ captions. Old scripted parity clips removed from the
doc (regenerable via scripts/render_metasim_1to1_gallery.py).

Author: geng-haoran