1. tuned arrow mesh drawing

2. centralixzed arrow config in layout.py
3. seperate mesh render and display resolution to speed up (~40 fps)

tests/test_contact_arrow_scale.py

@@ -0,0 +1,42 @@
+import sys
+from pathlib import Path
+
+_EXAMPLES = Path(__file__).resolve().parents[1] / "examples"
+if str(_EXAMPLES) not in sys.path:
+    sys.path.insert(0, str(_EXAMPLES))
+
+from viz.contact_arrow import (
+    ContactArrowStyle,
+    DEFAULT_CONTACT_ARROW_STYLE,
+    force_visual_strength,
+    length_from_force,
+    thickness_scale_from_force,
+)
+
+REFERENCE_NORMAL_FORCE = DEFAULT_CONTACT_ARROW_STYLE.reference_normal_force
+
+
+def test_force_visual_strength_monotonic():
+    forces = [0.0, 50.0, 175.0, 1000.0, 5000.0, REFERENCE_NORMAL_FORCE, 20000.0]
+    strengths = [force_visual_strength(f) for f in forces]
+    assert strengths[0] == 0.0
+    assert all(strengths[i] <= strengths[i + 1] for i in range(len(strengths) - 1))
+    assert strengths[-1] == 1.0
+
+
+def test_log_mode_more_sensitive_than_linear_at_low_force():
+    low = 175.0
+    log_style = ContactArrowStyle(force_strength_mode="log")
+    lin_style = ContactArrowStyle(force_strength_mode="linear")
+    assert force_visual_strength(low, style=log_style) > force_visual_strength(low, style=lin_style)
+
+
+def test_length_and_thickness_track_same_strength():
+    force = 2500.0
+    mesh_span = 10.0
+    strength = force_visual_strength(force)
+    length = length_from_force(force, mesh_span)
+    thick = thickness_scale_from_force(force)
+    assert length > mesh_span * 0.02
+    assert thick > 0.20
+    assert strength == force_visual_strength(force)