refactor(viewport): unify paper into one shader, transparent clear

Collapse the paper-space rendering path into the same shader widget the
model layout uses. The PaperCanvas widget draws the paper sheet, paper
entities and viewport borders underneath; the unified shader paints all
content viewports on top, scissored to each viewport's rect.

- Drop PaperViewportPane / PaperViewportPipeline newtype /
  PaperViewportPrimitive, ViewportPaneMode::Paper, ViewportPane::paper,
  build_primitive and build_viewport_primitive. Scene exposes only
  build_viewports.
- view.rs: the paper branch stacks PaperCanvas + the unified shader
  instead of the old paper_canvas_view function. The active paper
  viewport's blue border, render-mode picker and ViewCube hit-area are
  layered in the same viewport_stack as the model layout.
- Transparent shader clear (alpha=0) + alpha-blended blit pipeline so
  the underlying widget (container bg in model, PaperCanvas sheet in
  paper) shows through everywhere the shader has not painted geometry —
  including inside content viewport rectangles.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/pipeline/mod.rs

@@ -772,7 +772,12 @@ impl Pipeline {
                 entry_point: Some("fs_main"),
                 targets: &[Some(wgpu::ColorTargetState {
                     format,
-                    blend: None,
+                    // Alpha-blend the resolve texture onto the surface so a
+                    // transparent shader clear (alpha=0 outside any drawn
+                    // geometry) leaves whatever the underlying widget
+                    // painted — model container bg in a model layout,
+                    // PaperCanvas sheet in a paper layout — visible.
+                    blend: Some(wgpu::BlendState::ALPHA_BLENDING),
                     write_mask: wgpu::ColorWrites::ALL,
                 })],
                 compilation_options: wgpu::PipelineCompilationOptions::default(),