feat(paper): render the whole sheet via the shader, drop PaperCanvas

Paper space now renders entirely through the unified GPU shader, exactly
like model space — there is no separate 2-D canvas. The full-canvas
top-locked "sheet" viewport draws the layout's entities, hatches,
wipeouts and images; the desk is the container background and the white
printable area is a synthetic solid hatch.

- The white sheet fill uses draw_depth -2.0 (entity fills/wires are in
  the signed (-1, 1) range) so it is always strictly behind every object,
  with a negligible z offset (BIAS 0.001) — no far-plane clipping.
- Delete src/scene/paper_canvas.rs (the PaperCanvas widget) and its dead
  helpers (paper_canvas_wires / paper_sheet_wires / paper_canvas_cache).
  paper_canvas_hatches / _wipeouts remain — still used by paper hit-test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/render.rs

@@ -653,14 +653,22 @@ impl Scene {
             height: visible_h / canvas.1,
         };
 
-        // Stage 1: the paper sheet instance contributes only entity wires; its
-        // hatch / wipeout / image fills are still drawn by PaperCanvas beneath
-        // the shader. (Stage 2 moves those into the shader and drops PaperCanvas.)
-        let (hatches, wipeout_hatches, images) = if inst.paper_sheet {
-            (Arc::new(Vec::new()), Arc::new(Vec::new()), Arc::new(Vec::new()))
+        // The paper sheet instance renders the same hatch / wipeout / image
+        // fills as everything else (wrong-camera fills fall outside its
+        // frustum and are culled), plus a synthetic white fill for the sheet's
+        // printable area — the GPU equivalent of the old PaperCanvas chrome.
+        let hatches = if inst.paper_sheet {
+            let mut v: Vec<HatchModel> = Vec::new();
+            if let Some(sheet) = self.paper_sheet_fill() {
+                v.push(sheet);
+            }
+            v.extend(self.hatch_models_arc().iter().cloned());
+            Arc::new(v)
         } else {
-            (self.hatch_models_arc(), self.wipeout_models_arc(), self.images_arc())
+            self.hatch_models_arc()
         };
+        let wipeout_hatches = self.wipeout_models_arc();
+        let images = self.images_arc();
 
         Some(ViewportData {
             wires: all_wires,