Audit prop budgets and add a prop-gallery art check

- audit_glb decodes geometry: 400 tris / 600 verts budget, degenerate
  triangles, UV range, ground contact, one material; newspaper box parts moved
  to sit on y=0
- capture_prop_gallery.gd renders the six props on a studio backdrop; new
  prop_gallery catalogue entry + AI mockup (compare 10 good, judge match 0.90)
- headless test that instances share the imported atlas material
- README documents the gallery capture and budget; TODO props pipeline section
  moved to DONE

src/test/tests/test_generated_props.gd

@@ -68,6 +68,34 @@ func test_prop_scenes_share_one_atlas_material() -> bool:
 		instance.free()
 	return TestUtils.check(problems.is_empty(), "one atlas material per prop (%s)" % str(problems))
 
+func test_prop_instances_share_the_imported_material() -> bool:
+	## Instancing a prop must not duplicate its atlas material; every copy uses
+	## the same resource, so many props stay cheap to draw.
+	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 first := packed.instantiate()
+		var second := packed.instantiate()
+		var a := _first_material(first)
+		var b := _first_material(second)
+		if a == null or b == null:
+			problems.append("%s: missing material" % prop)
+		elif a.get_instance_id() != b.get_instance_id():
+			problems.append("%s: material is duplicated per instance" % prop)
+		first.free()
+		second.free()
+	return TestUtils.check(problems.is_empty(), "prop instances share materials (%s)" % str(problems))
+
+func _first_material(instance: Node) -> Material:
+	for node in instance.find_children("*", "MeshInstance3D", true, false):
+		var mesh: Mesh = (node as MeshInstance3D).mesh
+		if mesh != null and mesh.get_surface_count() > 0:
+			return mesh.surface_get_material(0)
+	return null
+
 func test_world_places_generated_props() -> bool:
 	var root := Node3D.new()
 	var builder := WorldBuilder.new(root)