feat: support explicit opt-in velocity observations for WidowX policies #260

Problem

Physical AI Studio currently records WidowX observations with joint positions only:

• Single arm: 7 values
• Bimanual: 14 values

Before #259, the Runtime WidowX drivers built their default policy state by concatenating joint positions and velocities:

• Single arm: 14 values
• Bimanual: 28 values

As a result, a policy trained from a current Studio dataset failed at inference because Runtime supplied a different observation.state width than the model was trained on.

#259 fixes the immediate problem by making the default WidowX policy state positions-only. Raw velocities are still collected and remain available through sensor_data["velocities"].

Solution

While with #259 we've fixed the runtime issue, ideally both library and studio should allow users to opt-in to record with velocities.
On the application side we could let users choose to record with velocities when setting up the robot (perhaps it can be part of the payload, added as a boolean in the constructor).
On the library side we need a better way to detect if a robot's state is compatible with a model. Right now the user only sees that a state vector of length 14 is not compatible with one of length 7.
Perhaps instead the model manifest can include a list of joint names that were used when recording the dataset, and check if these names are compatible with the joints used during inference.
This may require changing the send_action interface (e.g. changing action: np.ndarray to take an RobotObservation instead?) or we'd need to include this information from the robot's constructor?