feat(lingbot_va): RoboTwin eef-pose eval, single-file model, Hub checkpoints

Make the LingBot-VA port runnable on both LIBERO and RoboTwin and clean up the
package to LeRobot conventions.

- Consolidate all vendored Wan2.2 model code (transformer, attention, VAE helpers,
  flow-matching scheduler, grid utils, flex-attention) into a single
  modeling_lingbot_va.py; remove the separate wan_*/schedulers modules.
- Move the fixed action (un)normalization quantiles out of the config and into the
  post-processor (LIBERO 7-DoF + RoboTwin 16-d eef); remove the conversion script in
  favour of ready-to-use LeRobot-format checkpoints on the Hub.
- Fixes found via on-sim validation: undo LIBERO's 180-degree image flip
  (image_hflip), encode obs as a multi-frame streaming-VAE clip, reset the streaming
  VAE cache between episodes, run the transformer in config.dtype, lazy-load frozen
  VAE/UMT5 by subfolder with the text encoder on CPU.
- RoboTwin: add an end-effector-pose action mode to RoboTwinEnv (16-d per-arm
  xyz+quat+gripper deltas composed onto the initial eef pose, executed via CuRobo IK)
  and the robotwin_tshape latent layout (full-res head + half-res wrists via a second
  streaming VAE) with the upstream RoboTwin action quantiles + camera mapping.
- Predicted-video saving works for both benchmarks; docs + tests updated.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/policies/lingbot_va/test_modules.py

@@ -14,14 +14,20 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""Pure-torch unit tests for the vendored LingBot-VA helper modules (no diffusers needed)."""
+"""Unit tests for the vendored LingBot-VA helper code (scheduler + grid utilities)."""
 
 from __future__ import annotations
 
+import pytest
 import torch
 
-from lerobot.policies.lingbot_va.schedulers import FlowMatchScheduler
-from lerobot.policies.lingbot_va.wan_utils import data_seq_to_patch, get_mesh_id
+pytest.importorskip("diffusers")  # the model code lives in modeling_lingbot_va, which imports diffusers
+
+from lerobot.policies.lingbot_va.modeling_lingbot_va import (  # noqa: E402
+    FlowMatchScheduler,
+    data_seq_to_patch,
+    get_mesh_id,
+)
 
 
 def test_flow_match_scheduler_timesteps_monotone_decreasing() -> None: