WBC gait-clock variant

Gait-clock variant (WBCGaitPolicy)

NVIDIA's reference repo ships two MuJoCo G1 controllers. The non-gait pair (run_mujoco_gear_wbc.py: 86-dim observation, 7-wide command, a balance policy + a walk_policy selected by velocity) is WBCPolicy above. The gait-clock variant (run_mujoco_gear_wbc_gait.py) is WBCGaitPolicy:

• A single ONNX policy (no balance/walk split).
• A 95-dim observation frame, stacked x6 -> a [batch, 570] network input.
• An 8-wide command block with a freq_cmd step-frequency slot inserted at index 4 (so rpy moves to slots [5:8]), set via the gait_frequency kwarg.
• A 2-dim bipedal phase clock ([clock_FL, clock_FR]) appended to each frame - the locomotion rhythm the network steps to.

from strands_robots.policies import create_policy

policy = create_policy(
    "wbc_gait",                       # or the "sonic_gait" shorthand
    checkpoint="/path/to/gait-g1",    # a 95x6-input gait ONNX checkpoint
    target_velocity=[0.5, 0.0, 0.0],
    gait_frequency=1.5,               # freq_cmd (steps/s)
)
sim.run_policy(robot_name="unitree_g1", policy_object=policy,
               target_velocity=[0.5, 0.0, 0.0], gait_frequency=1.5,
               duration=5.0, control_frequency=50.0)

The shipped GR00T-WholeBodyControl-Balance.onnx / -Walk.onnx weights are the non-gait 516-wide family and do not load into this variant; supply a gait-clock checkpoint whose ONNX input is [batch, 570].

The new ingredient is the phase clock - a small stateful generator (GaitClock) that turns the velocity command + step frequency into the left/right-foot phase signal, with a walk-entry reseed, a warm-up ramp, and a static-stance freeze. It is a verbatim NumPy port of the upstream block (no torch) and is unit-tested against hand-computed values. examples/wbc_g1_gait.py --plot-clock visualizes it through a static -> walk -> static schedule (no checkpoint needed):

The WBCGaitPolicy bipedal phase clock (freq_cmd = 1.0): both feet are frozen at the held stance while static, then the left/right channels oscillate half a cycle out of phase during the walk window, returning to a frozen stance when the command goes static again. Produced by examples/wbc_g1_gait.py --plot-clock.