feat(theme): showcase harness — the human-eyeball proof

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

client/tests/unit/test_theme_showcase.gd

@@ -0,0 +1,11 @@
+# client/tests/unit/test_theme_showcase.gd
+extends "res://addons/gut/test.gd"
+
+
+func test_showcase_instances_without_error():
+	var packed = load("res://scenes/theme/theme_showcase.tscn")
+	assert_true(packed is PackedScene)
+	var node = packed.instantiate()
+	add_child_autofree(node)   # runs _ready(); any engine error here fails the suite
+	assert_true(node is Control)
+	assert_true(node.theme is Theme, "showcase did not apply game_theme")