fix(plot): text and leader background masks plot paper-white, not black

The mask is filled with the canvas colour; plotting carried that dark
colour (or the light-to-black rule) onto the sheet.

Fixes #1072

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/mod.rs

@@ -8573,7 +8573,7 @@ impl Scene {
     }
 
     /// Background colour for the current layout (model vs paper).
-    fn current_bg(&self) -> [f32; 4] {
+    pub(crate) fn current_bg(&self) -> [f32; 4] {
         if self.current_layout != "Model" {
             self.paper_bg_color
         } else {