fix(viz): gate arkit overlay on 2d freshness

persons_arkit_2d_t was written by IphoneUSBSource but never
read. A USB drop left the frozen skeleton on screen indefinitely.

- Add arkit_2d_fresh(ts_by_pid, now, max_age=1.0) pure helper
  in hand_display.py (8 TDD tests, all green)
- renderer._update_skeleton: use_arkit now requires at least one
  pid younger than 1 s (replaces bool check)
- Per-pid freshness gate in draw loop skips stale pids

data_only_viz/hand_display.py

@@ -30,6 +30,25 @@ class _HasXYC(Protocol):
     c: float
 
 
+def arkit_2d_fresh(ts_by_pid: dict, now: float, max_age: float = 1.0) -> bool:
+    """Return True if any pid has a timestamp younger than *max_age* seconds.
+
+    Args:
+        ts_by_pid: mapping pid -> perf_counter timestamp (from
+                   ``State.persons_arkit_2d_t``).
+        now:       current ``time.perf_counter()`` value.
+        max_age:   staleness threshold in seconds (exclusive: age < max_age).
+
+    Returns:
+        True if at least one entry satisfies ``now - ts < max_age``.
+        False if the dict is empty or every entry is stale.
+    """
+    for ts in ts_by_pid.values():
+        if (now - ts) < max_age:
+            return True
+    return False
+
+
 def hand_size(kp: list[_HasXYC]) -> float:
     """Euclidean distance wrist(0) -> middle-MCP(9) in normalised image units."""
     w = kp[0]

data_only_viz/renderer.py

@@ -44,7 +44,7 @@ from Metal import (
 from MetalKit import MTKView  # noqa: F401  (utilise par d'autres modules)
 
 from .arkit_skeleton import arkit_segments
-from .hand_display import hand_plausible, hand_size, segment_ok, panel_frame, panel_segments
+from .hand_display import arkit_2d_fresh, hand_plausible, hand_size, segment_ok, panel_frame, panel_segments
 from .mesh_topology import (
     BODY_TRIANGLES,
     FACE_TRIANGLES,
@@ -320,8 +320,9 @@ class MetalRenderer(NSObject):
 
         Priorise MediaPipe (body 33 + face 478 + 2 mains 21) si disponible
         et present ; sinon fallback COCO 17 keypoints YOLO."""
+        _arkit_now = time.perf_counter()
         use_arkit = (ARKIT_FULL and bool(s.arkit_parents)
-                     and bool(s.persons_arkit_2d))
+                     and arkit_2d_fresh(s.persons_arkit_2d_t, _arkit_now))
         if not use_arkit and not s.pose_alive():
             return 0
 
@@ -385,6 +386,9 @@ class MetalRenderer(NSObject):
         if use_arkit:
             parents = s.arkit_parents
             for pid, arr2d in s.persons_arkit_2d.items():
+                ts = s.persons_arkit_2d_t.get(pid)
+                if ts is None or (_arkit_now - ts) >= 1.0:
+                    continue  # skip stale or unknown pid
                 valid = s.persons_arkit_2d_valid.get(pid)
                 for (ax, ay, bx, by) in arkit_segments(arr2d, valid, parents):
                     if not push_seg(ax, ay, bx, by, 1.0, pid):

data_only_viz/tests/test_hand_display.py

@@ -10,6 +10,7 @@ from dataclasses import dataclass
 import pytest
 
 from data_only_viz.hand_display import (
+    arkit_2d_fresh,
     hand_plausible,
     hand_size,
     segment_ok,
@@ -345,3 +346,54 @@ def test_panel_frame_traces_rect():
                 for cx, cy in corners
             )
             assert on_corner, f"Endpoint ({ex:.4f}, {ey:.4f}) not a corner of panel rect"
+
+
+# ---------------------------------------------------------------------------
+# arkit_2d_fresh
+# ---------------------------------------------------------------------------
+
+def test_arkit_2d_fresh_empty_dict_is_false() -> None:
+    """No pids → never fresh."""
+    assert arkit_2d_fresh({}, now=1000.0) is False
+
+
+def test_arkit_2d_fresh_single_fresh_pid() -> None:
+    """A timestamp 0.5 s old with max_age=1.0 → fresh."""
+    ts_by_pid = {0: 999.5}
+    assert arkit_2d_fresh(ts_by_pid, now=1000.0) is True
+
+
+def test_arkit_2d_fresh_single_stale_pid() -> None:
+    """A timestamp 2 s old with max_age=1.0 → stale."""
+    ts_by_pid = {0: 998.0}
+    assert arkit_2d_fresh(ts_by_pid, now=1000.0) is False
+
+
+def test_arkit_2d_fresh_exactly_at_max_age_is_stale() -> None:
+    """age == max_age is NOT fresh (strict less-than)."""
+    ts_by_pid = {0: 999.0}
+    assert arkit_2d_fresh(ts_by_pid, now=1000.0, max_age=1.0) is False
+
+
+def test_arkit_2d_fresh_mix_stale_and_fresh() -> None:
+    """At least one fresh pid makes the whole dict fresh."""
+    ts_by_pid = {0: 990.0, 1: 999.8}  # pid 0 stale, pid 1 fresh
+    assert arkit_2d_fresh(ts_by_pid, now=1000.0) is True
+
+
+def test_arkit_2d_fresh_all_pids_stale() -> None:
+    """All pids stale → False."""
+    ts_by_pid = {0: 990.0, 1: 980.0}
+    assert arkit_2d_fresh(ts_by_pid, now=1000.0) is False
+
+
+def test_arkit_2d_fresh_custom_max_age() -> None:
+    """max_age=0.5 makes a 0.4 s old timestamp fresh."""
+    ts_by_pid = {5: 999.6}
+    assert arkit_2d_fresh(ts_by_pid, now=1000.0, max_age=0.5) is True
+
+
+def test_arkit_2d_fresh_custom_max_age_stale() -> None:
+    """max_age=0.5 makes a 0.6 s old timestamp stale."""
+    ts_by_pid = {5: 999.4}
+    assert arkit_2d_fresh(ts_by_pid, now=1000.0, max_age=0.5) is False