test(viz): pin arkit max-len behavior change

Review follow-up: make the v2 filter's rejection of legacy 0.707
diagonal fixtures explicit instead of silently retuned, and document
the close-up caveat on max_len (ARKIT_BONE_MAX raises it live).

data_only_viz/arkit_skeleton.py

@@ -23,6 +23,9 @@ def arkit_segments(arr2d, valid, parents, max_len: float = 0.5, bounds: float =
     parents: parent index per joint (-1 = root).
     max_len: maximum bone length in normalized units; bones longer than this
              are skipped (garbage coords during ARKit tracking loss).
+             Caveat: on an extreme close-up (subject filling the frame) a
+             legit bone can approach 0.5 — raise ARKIT_BONE_MAX live if
+             the skeleton visibly loses bones when very near the camera.
     bounds:  margin outside [0,1] that is still accepted; endpoints outside
              [-bounds, 1+bounds] in x or y are rejected.
     """

data_only_viz/tests/test_arkit_skeleton.py

@@ -80,3 +80,16 @@ def test_segments_in_bounds_endpoint_passes():
     parents = [-1, 0]
     segs = arkit_segments(arr2d, valid, parents, bounds=0.1)
     assert len(segs) == 1
+
+
+def test_segments_legacy_diagonal_bone_now_filtered():
+    """Regression pin: the pre-filter test fixtures used a full-diagonal
+    bone (0,0)->(0.5,0.5), length ~0.707. The max_len=0.5 default now
+    REJECTS it — this is the intended behavior change of the v2 filter,
+    made explicit here instead of silently retuning old fixtures."""
+    arr2d = [(0.0, 0.0), (0.5, 0.5)]
+    valid = [True, True]
+    parents = [-1, 0]
+    assert arkit_segments(arr2d, valid, parents) == []
+    # ...and the env-tunable knob restores it for close-up shots.
+    assert len(arkit_segments(arr2d, valid, parents, max_len=0.8)) == 1