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/assets/theme/shaders/parchment.gdshader

@@ -0,0 +1,6 @@
+shader_type canvas_item;
+uniform vec4 top : source_color;
+uniform vec4 bottom : source_color;
+void fragment() {
+	COLOR = mix(top, bottom, UV.y);
+}