feat(paper): render the sheet as a top-locked GPU viewport

Paper space now renders its sheet through the unified shader, the same
path as model space: a full-canvas "sheet" ViewportInstance draws the
layout's own entities + viewport borders + interim/preview wires, with a
camera locked to the top/plan orientation (paper is 2-D and never
orbits). Floating content viewports overlay it.

- PaperCanvas no longer draws entity wires (still paints the desk, sheet
  and hatch/wipeout/image fills; full removal is the next step).
- Right-drag on the paper sheet no longer orbits the camera (it would
  corrupt the frame and skew subsequent pans).
- Fix floating-viewport orbit oscillation: orbit_active_viewport wrote
  view_direction.y negated while camera_for_viewport reads it un-negated,
  so each drag step read back a Y-mirrored camera and flipped the model
  between a rotation and its opposite every frame. Store eye directly,
  matching snap_active_viewport_to_direction and the reader.

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

src/app/update.rs

@@ -1951,12 +1951,18 @@ impl OpenCADStudio {
                                 drop(sel);
                                 self.tabs[i].scene.orbit_active_viewport(dx, dy);
                                 return Task::none();
-                            } else {
+                            } else if self.tabs[i].scene.current_layout == "Model" {
                                 sel.right_last_pos = Some(p);
                                 drop(sel);
                                 self.tabs[i].scene.camera.borrow_mut().orbit(dx, dy);
                                 self.tabs[i].scene.camera_generation += 1;
                                 return Task::none();
+                            } else {
+                                // Paper sheet is top-locked: right-drag never
+                                // orbits it (orbiting would corrupt the camera
+                                // frame and skew subsequent pans).
+                                sel.right_last_pos = Some(p);
+                                return Task::none();
                             }
                         } else {
                             sel.right_last_pos = Some(p);