feat(viz): bold body skeleton strokes

Metal lines are 1px and the skeleton was barely visible in
performance. Each ARKit bone is now drawn in concentric offset passes
(SKEL_LINE_WIDTH, default 4 approx px; 1 restores the thin look).

data_only_viz/config.py

@@ -45,6 +45,7 @@ VIZ_HUD                     "0"       bool   show debug HUD overlay
 VIZ_SOURCE_KEYS             "0"       bool   wxcvbn + 0-9 source-bundle shortcuts (off: perf safety)
 VIZ_AUDIO_KEYS              "0"       bool   qsdfghjkl audio-scene shortcuts (off: perf safety; m stays)
 VIZ_DEFAULT_MODE            "auto"    str    fixed boot viz mode (e.g. voronoi); disables auto-openpos
+SKEL_LINE_WIDTH             "4"       int    body skeleton stroke width, approx px (1 = thin)
 IPHONE_OSC_PORT             57128     int    UDP port for iPhone ARBodyTracker /body3d/kp
 CONCERT_MIRROR              "1"       bool   mirror video horizontally (!=0 = True)
 ICP_FUSION                  "0"       bool   enable ICP LiDAR fusion (only "1" = True)
@@ -147,6 +148,9 @@ class VizConfig:
     # appear). A fixed mode also DISABLES the auto-openpos timer so it is
     # truly always on (user request live 2026-07-02: t/voronoi permanent).
     viz_default_mode: str = "auto"
+    # Approximate body-skeleton stroke width in pixels (multi-pass offsets;
+    # Metal lines are 1px). 1 = thin historical look.
+    skel_line_width: int = 4
 
     # ---- iPhone OSC ------------------------------------------------------
     iphone_osc_port: int = 57128
@@ -297,6 +301,7 @@ class VizConfig:
             viz_source_keys=_bool_std("VIZ_SOURCE_KEYS", False),
             viz_audio_keys=_bool_std("VIZ_AUDIO_KEYS", False),
             viz_default_mode=_str("VIZ_DEFAULT_MODE", "auto"),
+            skel_line_width=_int("SKEL_LINE_WIDTH", 4),
             iphone_osc_port=_int("IPHONE_OSC_PORT", 57128),
             concert_mirror=_bool_ne0("CONCERT_MIRROR", True),
             # ICP_FUSION / MULTIHMR_AUTOCAST use strict "1" convention

data_only_viz/renderer.py

@@ -171,6 +171,7 @@ class MetalRenderer(NSObject):
         # future iPhone app build flips it.
         self._hand_swap_lr = _cfg.hand_swap_lr
         self._arkit_full = _cfg.arkit_full_skeleton
+        self._skel_width = max(1, int(_cfg.skel_line_width))
         self._init_skel_cpu_buffer()
         self._init_mesh_cpu_buffer()
         self._build_pipelines()
@@ -432,11 +433,21 @@ class MetalRenderer(NSObject):
                         valid = ~fmask
                     elif len(valid) == len(fmask):
                         valid = valid & ~fmask
+                # Bold strokes: Metal lines are 1px, so each bone is drawn
+                # in concentric offset passes (SKEL_LINE_WIDTH approx px).
+                _w = getattr(self, "_skel_width", 1)
+                _o = 1.0 / max(1, s.height or 720)
+                _offsets = [(0.0, 0.0)]
+                for _r in range(1, _w):
+                    _d = _r * _o
+                    _offsets += [(_d, 0.0), (-_d, 0.0), (0.0, _d), (0.0, -_d)]
                 for (ax, ay, bx, by) in arkit_segments(
                     arr2d, valid, parents, max_len=self._arkit_bone_max
                 ):
-                    if not push_seg(ax, ay, bx, by, 1.0, pid):
+                    for _dx, _dy in _offsets:
-                        break
+                        if not push_seg(ax + _dx, ay + _dy,
+                                        bx + _dx, by + _dy, 1.0, pid):
+                            break
 
         if self._mp_bones is not None and (
             s.persons_body or s.persons_face or s.persons_hands or

data_only_viz/tests/test_config.py

@@ -425,3 +425,8 @@ def test_viz_default_mode():
     assert VizConfig.from_env({}).viz_default_mode == "auto"
     assert VizConfig.from_env(
         {"VIZ_DEFAULT_MODE": "voronoi"}).viz_default_mode == "voronoi"
+
+
+def test_skel_line_width_default():
+    assert VizConfig.from_env({}).skel_line_width == 4
+    assert VizConfig.from_env({"SKEL_LINE_WIDTH": "1"}).skel_line_width == 1