feat(theme): parchment/dark-bay/vignette shader surfaces as drop-in scenes

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/tests/unit/test_surfaces.gd

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+extends "res://addons/gut/test.gd"
+
+const SCENES := [
+	"res://scenes/theme/surfaces/ParchmentPanel.tscn",
+	"res://scenes/theme/surfaces/DarkBay.tscn",
+	"res://scenes/theme/surfaces/VignetteOverlay.tscn",
+]
+
+
+func test_surfaces_instance_with_shader_material():
+	for path in SCENES:
+		var packed = load(path)
+		assert_true(packed is PackedScene, "not a scene: %s" % path)
+		var node = packed.instantiate()
+		add_child_autofree(node)   # triggers _ready()
+		assert_true(node.material is ShaderMaterial, "%s has no ShaderMaterial" % path)
+		assert_true(node.material.shader is Shader, "%s shader missing" % path)

client/tests/unit/test_surfaces.gd.uid

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+uid://csc3r7lqoe83d