Draw prop models with a single atlas material

Each generated prop used one material per part (body, accent, metal...). The
model pipeline now packs a prop's palette into one power-of-two colour atlas plus a
matching metallic/roughness atlas and emits a single material:

- atlas_for(): 8x8 cells per palette role, colours in sRGB, roughness in the G and
  metallic in the B channel of the second atlas; NEAREST sampling keeps cells flat.
- build_glb(): every part merges into one primitive with constant per-vertex atlas
  UVs; the material uses baseColorTexture + metallicRoughnessTexture with factors 1.0.
- audit_glb(): fails unless a prop has exactly one material, both atlas textures,
  two images, and TEXCOORD_0 on every primitive.
- Removed the now-unused sRGB->linear factor helpers; colour travels in the texture.

Verification: headless 181 -> 182 (new "one material per prop scene" test), Python
tooling 53/53 (atlas determinism/size/UV/palette-override tests replace the old
factor tests), model audit OK for all six props, screenshots regenerated and
screen_designs.py compare still 6 good / 0 bad. Version 0.3.1.

src/test/tests/test_generated_props.gd

@@ -39,6 +39,35 @@ func test_prop_scenes_instantiate_with_meshes() -> bool:
 		instance.free()
 	return TestUtils.check(problems.is_empty(), "every prop scene has geometry (%s)" % str(problems))
 
+func test_prop_scenes_share_one_atlas_material() -> bool:
+	## All parts of a prop share one material backed by the colour atlas, so a
+	## prop is a single draw material instead of one per part.
+	var problems: Array[String] = []
+	for prop in WorldBuilder.MODEL_PROPS:
+		var packed := ResourceLoader.load(MODEL_DIR + str(prop) + ".glb") as PackedScene
+		if packed == null:
+			problems.append("%s: no scene" % prop)
+			continue
+		var instance := packed.instantiate()
+		var meshes := instance.find_children("*", "MeshInstance3D", true, false)
+		var materials := {}
+		var surfaces := 0
+		for node in meshes:
+			var mesh: Mesh = (node as MeshInstance3D).mesh
+			if mesh == null:
+				continue
+			for index in mesh.get_surface_count():
+				surfaces += 1
+				var material := mesh.surface_get_material(index)
+				if material != null:
+					materials[material.get_instance_id()] = true
+		if surfaces == 0:
+			problems.append("%s: no surfaces" % prop)
+		elif materials.size() != 1:
+			problems.append("%s: %d materials (expected 1)" % [prop, materials.size()])
+		instance.free()
+	return TestUtils.check(problems.is_empty(), "one atlas material per prop (%s)" % str(problems))
+
 func test_world_places_generated_props() -> bool:
 	var root := Node3D.new()
 	var builder := WorldBuilder.new(root)