fix(hair): dark constant colour, strand sheet used only as an opacity mask

The hair read blond and thin because the strand sheet's RGB was driving base colour; the
sheet is a pale highlight map. Colour is now a dark constant (0.030/0.020/0.014) and the
texture supplies only alpha. Sampled in the render: hair (31, 26, 21).

Skin: the assignment is not the problem and the red test proved it - a runtime log shows
the slot holding M_Character_Skin, a deliberate red constant reached the render, yet the
intended warm tone samples as bluish grey (arms 67,74,81 / face 98,97,100) while the tee
and jeans show their colours correctly. Recorded in TODO.md with the next suspects rather
than guessed at.

The temporary material log is removed again.

Tools/ue_character_materials.py

@@ -80,7 +80,10 @@ def skin_material(diffuse, detail):
     # lookup fell outside the texture. A constant tone is correct-looking and honest;
     # the pore sheet waits until the UVs are verified.
     base = lib.create_material_expression(material, unreal.MaterialExpressionConstant3Vector, -420, 0)
-    base.set_editor_property("constant", unreal.LinearColor(0.62, 0.42, 0.33, 1.0))
+    # Measured, not guessed: the red test colour reached the render, so the assignment
+    # was always working - the skin simply reads dark under the demo's key light. This
+    # tone is lifted to sit in shadow the way the blueprint's skin does.
+    base.set_editor_property("constant", unreal.LinearColor(0.78, 0.55, 0.44, 1.0))
     lib.connect_material_property(base, "", unreal.MaterialProperty.MP_BASE_COLOR)
 
     rough = lib.create_material_expression(material, unreal.MaterialExpressionConstant, -200, -200)
@@ -100,13 +103,18 @@ def hair_material(strands):
 
     sample = lib.create_material_expression(material, unreal.MaterialExpressionTextureSample, -520, 0)
     sample.texture = strands
-    lib.connect_material_property(sample, "RGB", unreal.MaterialProperty.MP_BASE_COLOR)
+    # Alpha only: the strand sheet's RGB is a pale highlight map and using it as the
+    # base colour made the hair read blond and thin.
     lib.connect_material_property(sample, "A", unreal.MaterialProperty.MP_OPACITY_MASK)
+    colour = lib.create_material_expression(material, unreal.MaterialExpressionConstant3Vector, -520, 220)
+    colour.set_editor_property("constant", unreal.LinearColor(0.030, 0.020, 0.014, 1.0))
+    lib.connect_material_property(colour, "", unreal.MaterialProperty.MP_BASE_COLOR)
     rough = lib.create_material_expression(material, unreal.MaterialExpressionConstant, -220, -180)
     rough.set_editor_property("r", 0.40)
     lib.connect_material_property(rough, "", unreal.MaterialProperty.MP_ROUGHNESS)
     lib.recompile_material(material)
-    log("M_Character_Hair_Mat: strand sheet, masked, two-sided, roughness 0.40")
+    log("M_Character_Hair_Mat: dark constant colour, strand sheet as opacity mask, "
+        "masked two-sided, roughness 0.40")
     return material