feat(unitree_g1): use captured neutral SONIC token instead of zeros

The all-zero token is off the encoder's learned FSQ manifold and decodes to a
slightly goofy stance. Replace it with a NEUTRAL_TOKEN captured from the encoder's
own idle output in sim (stored as integer FSQ codes, rescaled by the encoder's
1/16 quantization step to an exact on-grid token). token_mode now seeds this
neutral, and the onboard sender starts observation.state from it so the first
inference sees the token the decoder is actually holding.

src/lerobot/robots/unitree_g1/controllers/sonic_whole_body.py

@@ -77,6 +77,22 @@ logger = logging.getLogger(__name__)
 # eases in without a snap on the first command.
 INIT_RAMP_S = 3.0
 
+# Neutral ("zero pose") SONIC token, held by token_mode until the first real token
+# arrives. Captured from the encoder's own output while the robot stood idle in sim
+# (capture_neutral_token.py): the encoder is an FSQ bottleneck (~5 bit/dim, 15.5 half-
+# width, Div(16)), so its tokens live on the 1/16 grid. We store the integer FSQ codes
+# and rescale by the same 1/16 step, giving an exact on-grid token -- unlike the literal
+# all-zero token, which is off the encoder's learned manifold and decodes to a slightly
+# goofy stance. This one decodes to a stable, natural standing pose.
+_NEUTRAL_TOKEN_CODES = np.array(
+    [-1, 3, 1, -1, 1, -3, 6, 1, 1, 1, -2, -4, -2, 0, -3, -1,
+     2, -1, -3, -5, 3, 1, 1, -4, -1, -1, 1, -7, 0, 1, 2, -2,
+     5, -2, -2, -4, 0, -1, 3, -1, 0, -5, -1, 0, -4, 0, 0, -1,
+     -1, 2, -2, 1, 3, 3, 1, 0, 0, 6, 0, -7, 3, 0, 2, -2],
+    dtype=np.float32,
+)
+NEUTRAL_TOKEN = _NEUTRAL_TOKEN_CODES / 16.0  # FSQ Div(16): integer codes -> on-grid token
+
 
 def _extract_wb34_from_action(action: dict | None) -> np.ndarray | None:
     """Reassemble a dense (34,) whole-body command from ``wb.{i}.pos`` keys, or None.
@@ -359,9 +375,10 @@ class SonicWholeBodyController:
         if token is not None:
             self._last_token = token
         elif self._last_token is None and self.token_mode:
-            # Token-driven deploy, but no token has arrived yet: hold the neutral
+            # Token-driven deploy, but no token has arrived yet: hold the captured
-            # (all-zero) token, which the decoder maps to a stable neutral stance.
+            # neutral token (NEUTRAL_TOKEN), which the decoder maps to a stable, natural
-            self._last_token = np.zeros(TOKEN_DIM, dtype=np.float32)
+            # standing pose (the encoder's own idle output; see NEUTRAL_TOKEN).
+            self._last_token = NEUTRAL_TOKEN.copy()
         if self._last_token is not None:
             # Either a fresh token this tick or the last one received (held between the
             # ~30 Hz token stream and the ~50 Hz control loop).

src/lerobot/robots/unitree_g1/infer_sonic_g1_onboard.py

@@ -56,7 +56,11 @@ from lerobot.cameras.zmq import ZMQCamera, ZMQCameraConfig
 from lerobot.configs.policies import PreTrainedConfig
 from lerobot.policies.factory import get_policy_class, make_pre_post_processors
 from lerobot.policies.utils import prepare_observation_for_inference
-from lerobot.robots.unitree_g1.controllers.sonic_whole_body import TOKEN_DIM, token_action_key
+from lerobot.robots.unitree_g1.controllers.sonic_whole_body import (
+    NEUTRAL_TOKEN,
+    TOKEN_DIM,
+    token_action_key,
+)
 
 logging.basicConfig(level=logging.INFO, format="%(asctime)s [%(levelname)s] %(message)s", force=True)
 logger = logging.getLogger("sonic_sender")
@@ -130,8 +134,9 @@ def main() -> None:
     signal.signal(signal.SIGTERM, lambda *_: stop.__setitem__("flag", True))
 
     # observation.state = the token currently executing on the robot (last one we sent);
-    # start at zeros, matching the decoder's zero-seeded initial state.
+    # start at the neutral token the decoder holds before the first send, so the very
-    prev_token = np.zeros(TOKEN_DIM, dtype=np.float32)
+    # first inference sees the true executing token (not zeros).
+    prev_token = NEUTRAL_TOKEN.copy()
     period = 1.0 / args.fps
     n = 0
     t_infer_total = 0.0