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/scripts/theme/surfaces/parchment_panel.gd

@@ -0,0 +1,12 @@
+class_name ParchmentPanel
+extends Panel
+## Drop-in parchment backdrop (mockups "Dark Bay + Parchment" motif). §2: presentation.
+const Palette = preload("res://scripts/theme/palette.gd")
+
+
+func _ready() -> void:
+	var m := ShaderMaterial.new()
+	m.shader = load("res://assets/theme/shaders/parchment.gdshader")
+	m.set_shader_parameter("top", Palette.SHEET_TOP)
+	m.set_shader_parameter("bottom", Palette.SHEET_BOTTOM)
+	material = m