feat(annotate): Module 1 sees the whole episode as one video block

Replaces keyframe sampling with a single Qwen-VL video block covering
the whole demonstration. The model pools temporally itself and chooses
where to cut subtasks — no stride, no count, no keyframe count knob to
tune.

- frames.py: ``FrameProvider`` gains ``video_for_episode(record,
  max_frames)``; ``VideoFrameProvider`` samples up to ``max_frames``
  uniformly across the episode duration; ``_NullProvider`` returns []
  for the no-video fallback. New ``to_video_block`` helper.
- Module 1: drops keyframe sampling. The subtask prompt now goes out as
  ``[{"type":"video", "video":[<frames>]}, {"type":"text", ...}]`` and
  the prompt template asks the model to "watch the whole clip, then
  segment it" with cut points decided from gripper/contact/regrasp
  events the model sees.
- Module1Config: ``keyframes_per_episode`` removed; replaced with
  ``max_video_frames: int = 32`` (model-capacity bound, not annotation
  logic).
- Test: ``test_module1_attaches_video_block_to_subtask_prompt`` locks in
  the single-video-block invariant.
- Stub-VLM markers updated: tests now key on "atomic subtasks" instead
  of the old "Decompose the demonstration" phrase that no longer
  appears in the prompt.
- Docs: updated to describe the whole-episode video-block behavior and
  the no-video fallback.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/source/annotation_pipeline.mdx

@@ -36,11 +36,20 @@ uv run lerobot-annotate \
   --vlm.tensor_parallel_size=2
 ```
 
-The pipeline attaches camera keyframes to every Module 1/2/3 prompt by
-default, decoded from the dataset's first `observation.images.*` stream.
-Override with `--vlm.camera_key=observation.images.<name>` to pin a
-specific viewpoint. Datasets with no video tracks fall back to text-only
-prompts automatically.
+The pipeline attaches actual camera footage to every Module 1/2/3 prompt
+by default, decoded from the dataset's first `observation.images.*`
+stream. Override with `--vlm.camera_key=observation.images.<name>` to
+pin a specific viewpoint. Datasets with no video tracks fall back to
+text-only prompts automatically.
+
+**Module 1 sees the whole episode as one video block.** Subtask
+decomposition gets a `{"type":"video", "video":[<frames>]}` block
+covering the entire demonstration; Qwen-VL pools temporally on its own
+and decides where to cut. There is no keyframe stride or count knob —
+`--module_1.max_video_frames` (default 32) only caps the frames packed
+into the video block as a model-capacity bound. Module 2 attaches a
+single still frame at the interjection timestamp; Module 3 attaches the
+exact emission frame to each VQA pair.
 
 The executor picks `LocalPipelineExecutor` for small datasets and
 `SlurmPipelineExecutor` for large ones based on