feat: introduce ViewportPane widget for unified model/paper rendering

- Remove Scene: shader::Program<Msg> impl; all GPU rendering now goes
  through ViewportPane which supports three modes:
    Model      — full model-space view (unchanged behaviour)
    PaperSheet — paper-space entities only, using the paper camera
    Paper      — model content through a specific viewport's own camera

- Add Scene helper methods (render.rs):
    build_primitive / build_paper_sheet_primitive / build_viewport_primitive
    update_viewcube_state / viewcube_mouse_interaction

- Add Scene helpers (mod.rs):
    paper_sheet_wires()        — paper entities without viewport projection
    camera_for_viewport()      — derive Camera from Viewport entity data
    model_wires_for_viewport() — model wires filtered by per-viewport frozen layers
    viewport_screen_rect()     — paper-space → pixel rect mapping

- Add paper_canvas_view() in view.rs:
    Stack of PaperSheet + one Paper widget per Viewport entity,
    positioned with Space offsets from viewport_screen_rect()

- Update VIEWPORT_WIDGET_PLAN.md: mark Steps 1–4 done, document remaining work

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/view.rs

@@ -4,6 +4,7 @@ use super::history::history_dropdown_labels;
 use super::helpers::grid_plane_from_camera;
 use crate::scene::{VIEWCUBE_DRAW_PX, VIEWCUBE_PAD};
 use crate::scene::grip::grips_to_screen;
+use crate::scene::viewport_pane::ViewportPane;
 use crate::ui::overlay;
 use iced::widget::{button, column, container, mouse_area, row, shader, stack, text, Row, Space};
 use iced::window;
@@ -103,7 +104,14 @@ impl H7CAD {
         let i = self.active_tab;
         let tab = &self.tabs[i];
         let is_paper = tab.scene.current_layout != "Model";
-        let viewport_3d = shader(&tab.scene).width(Fill).height(Fill);
+        let viewport_3d: Element<'_, Message> = if is_paper {
+            paper_canvas_view(tab)
+        } else {
+            shader(ViewportPane::model(&tab.scene))
+                .width(Fill)
+                .height(Fill)
+                .into()
+        };
 
         let selection_overlay = {
             let sel = tab.scene.selection.borrow().clone();
@@ -447,6 +455,65 @@ impl H7CAD {
     }
 }
 
+// ── Paper canvas ──────────────────────────────────────────────────────────
+//
+// In paper space, the main canvas is composed of:
+//   1. A full-size PaperSheet layer — renders paper-space entities (title blocks,
+//      frames, borders) with the paper-space camera.
+//   2. One ViewportPane::Paper widget per Viewport entity, positioned at its
+//      paper-space coordinates and sized to its paper-space dimensions.
+//      Each renders model-space content through its own viewport camera.
+//   3. The existing selection_overlay / viewport_mouse / nav stack on top
+//      (unchanged — mouse event routing stays the same).
+
+fn paper_canvas_view<'a>(tab: &'a super::document::DocumentTab) -> Element<'a, Message> {
+    let scene = &tab.scene;
+
+    // Layer 1: paper-space entities (title blocks, frames, paper boundary).
+    let paper_sheet = shader(ViewportPane::paper_sheet(scene))
+        .width(Fill)
+        .height(Fill);
+
+    // Use the last-rendered canvas size to compute pixel positions.
+    // On the very first frame this is (0,0); correct positions appear from
+    // the second frame onward (imperceptible in practice).
+    let (canvas_w, canvas_h) = scene.selection.borrow().vp_size;
+    let vp_list = scene.viewport_list();
+
+    let mut layers = stack![paper_sheet];
+
+    for (handle, _, _) in &vp_list {
+        let Some(rect) = scene.viewport_screen_rect(*handle, (canvas_w, canvas_h)) else {
+            continue;
+        };
+
+        // Clamp to avoid zero-size or negative widgets.
+        let w = rect.width.max(1.0);
+        let h = rect.height.max(1.0);
+        let x = rect.x.max(0.0);
+        let y = rect.y.max(0.0);
+
+        // Layer 2+: one shader widget per viewport, positioned with Space offsets.
+        let vp_widget = shader(ViewportPane::paper(scene, *handle))
+            .width(iced::Length::Fixed(w))
+            .height(iced::Length::Fixed(h));
+
+        let positioned = column![
+            Space::new().height(iced::Length::Fixed(y)),
+            row![
+                Space::new().width(iced::Length::Fixed(x)),
+                vp_widget,
+            ],
+        ]
+        .width(Fill)
+        .height(Fill);
+
+        layers = layers.push(positioned);
+    }
+
+    layers.width(Fill).height(Fill).into()
+}
+
 // ── Document tab bar ───────────────────────────────────────────────────────
 
 pub(super) fn doc_tab_bar<'a>(tabs: &'a [DocumentTab], active_tab: usize) -> Element<'a, Message> {