feat(pose): decode iphone 2D skeleton

Add TAG_SKELETON2D=6, SKEL2D_BYTES=819, decode_skeleton2D() to
iphone_usb_bridge.py; handler in IphoneUSBSource writing
persons_arkit_2d / persons_arkit_2d_t to state; new state fields
in State dataclass. TDD: test_arkit_body.py (round-trip + wrong
length → None). Import check verified.

data_only_viz/iphone_usb_source.py

@@ -18,7 +18,8 @@ import cv2
 import numpy as np
 
 from data_only_viz.scripts.iphone_usb_bridge import (
-    connect_device, decode_skeleton, iter_frames, TAG_SKELETON,
+    connect_device, decode_skeleton, decode_skeleton2D,
+    iter_frames, TAG_SKELETON, TAG_SKELETON2D,
 )
 from data_only_viz.state import PoseKp
 
@@ -148,6 +149,13 @@ class IphoneUSBSource:
                                         arr[i] = (x, y, z)
                                 self.state.persons_arkit_joints[pid] = arr
                                 self.state.persons_arkit_last_t[pid] = time.perf_counter()
+                    elif tag == TAG_SKELETON2D and self.state is not None:
+                        pts = decode_skeleton2D(payload)
+                        if pts is not None:
+                            arr = np.array([[x, y] for (x, y, _v) in pts], dtype=np.float32)
+                            with self.state.lock():
+                                self.state.persons_arkit_2d[pid] = arr
+                                self.state.persons_arkit_2d_t[pid] = time.perf_counter()
                     elif tag == TAG_HANDS:
                         hands = _decode_hands(payload)
                         if hands is not None and self.state is not None:

data_only_viz/scripts/iphone_usb_bridge.py

@@ -44,6 +44,9 @@ TAG_SKELETON = 1
 JOINT_COUNT = 91
 SKEL_FLOAT_BYTES = JOINT_COUNT * 3 * 4          # 1092
 SKEL_BYTES = SKEL_FLOAT_BYTES + JOINT_COUNT     # 1183
+TAG_SKELETON2D = 6                               # wire: skeleton2D (face=5 taken)
+SKEL2D_FLOAT_BYTES = JOINT_COUNT * 2 * 4        # 728
+SKEL2D_BYTES = SKEL2D_FLOAT_BYTES + JOINT_COUNT # 819
 MAX_PAYLOAD = 8 * 1024 * 1024
 
 LOG = logging.getLogger("iphone_usb_bridge")
@@ -169,6 +172,19 @@ def decode_skeleton(payload: bytes):
     ]
 
 
+def decode_skeleton2D(payload: bytes):
+    """Return [(x, y, valid), ...] of length 91 (normalized 0..1), or None.
+
+    Wire layout: 91 × (x f32 BE, y f32 BE) = 728 bytes, then 91 validity bytes.
+    Total: SKEL2D_BYTES = 819.
+    """
+    if len(payload) != SKEL2D_BYTES:
+        return None
+    floats = struct.unpack(">" + "f" * (JOINT_COUNT * 2), payload[:SKEL2D_FLOAT_BYTES])
+    valid = payload[SKEL2D_FLOAT_BYTES:]
+    return [(floats[i * 2], floats[i * 2 + 1], valid[i] != 0) for i in range(JOINT_COUNT)]
+
+
 # --------------------------------------------------------------------------
 # main loop
 # --------------------------------------------------------------------------

data_only_viz/state.py

@@ -144,6 +144,10 @@ class State:
     # (metres, hip-centered). Fresh = updated within < 1 s.
     persons_arkit_joints: dict = field(default_factory=dict)
     persons_arkit_last_t: dict = field(default_factory=dict)
+    # ARKit 2D projected skeleton (Task 3): 91×2 float32 arrays (normalized
+    # screen coords 0..1) per pid. Updated by IphoneUSBSource on TAG_SKELETON2D.
+    persons_arkit_2d: dict[int, "np.ndarray"] = field(default_factory=dict)
+    persons_arkit_2d_t: dict[int, float] = field(default_factory=dict)
 
     # ---- LiDAR / ICP mesh fusion (Task 8 - 2026-05-14) ----
     # Set by the LidarTCPReader poller; consumed by FusionWorker.run_once.

data_only_viz/tests/test_arkit_body.py

@@ -0,0 +1,29 @@
+"""Task 3: decode_skeleton2D round-trip tests.
+
+Wire layout (TAG_SKELETON2D = 6):
+    91 joints * (x: f32 BE, y: f32 BE) = 728 bytes of floats
+    + 91 validity bytes
+    = 819 bytes total
+"""
+import struct
+
+from data_only_viz.scripts.iphone_usb_bridge import decode_skeleton2D, JOINT_COUNT
+
+
+def test_decode_skeleton2d_roundtrip():
+    floats = []
+    for i in range(JOINT_COUNT):
+        floats += [i * 0.001, i * 0.002]
+    payload = struct.pack(">" + "f" * (JOINT_COUNT * 2), *floats)
+    payload += bytes(1 if i % 2 == 0 else 0 for i in range(JOINT_COUNT))
+    out = decode_skeleton2D(payload)
+    assert out is not None and len(out) == JOINT_COUNT
+    assert abs(out[10][0] - 0.010) < 1e-5 and out[10][2] is True
+    assert out[11][2] is False
+
+
+def test_decode_skeleton2d_wrong_length_returns_none():
+    assert decode_skeleton2D(b"\x00" * 10) is None
+    assert decode_skeleton2D(b"\x00" * 818) is None
+    assert decode_skeleton2D(b"\x00" * 820) is None
+    assert decode_skeleton2D(b"") is None