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 <noreply@anthropic.com>

src/scene/viewport_pane.rs

@@ -0,0 +1,94 @@
+use super::render::{CameraState, Primitive};
+use super::Scene;
+use acadrust::Handle;
+use iced::widget::shader;
+use iced::{mouse, Event, Rectangle};
+
+// ── Mode ──────────────────────────────────────────────────────────────────
+
+pub enum ViewportPaneMode {
+    /// Full model space — fills whatever bounds Iced assigns.
+    Model,
+    /// Paper-space entities only (title blocks, frames, borders) using the
+    /// paper-space camera. No viewport content projection.
+    PaperSheet,
+    /// Model-space content seen through a specific paper-space Viewport entity.
+    Paper { handle: Handle },
+}
+
+// ── Widget struct ─────────────────────────────────────────────────────────
+
+pub struct ViewportPane<'a> {
+    pub scene: &'a Scene,
+    pub mode: ViewportPaneMode,
+}
+
+impl<'a> ViewportPane<'a> {
+    pub fn model(scene: &'a Scene) -> Self {
+        Self { scene, mode: ViewportPaneMode::Model }
+    }
+
+    /// Paper-sheet layer: paper-space entities rendered with the paper camera.
+    pub fn paper_sheet(scene: &'a Scene) -> Self {
+        Self { scene, mode: ViewportPaneMode::PaperSheet }
+    }
+
+    /// One paper-space viewport: model content rendered through its own camera.
+    pub fn paper(scene: &'a Scene, handle: Handle) -> Self {
+        Self { scene, mode: ViewportPaneMode::Paper { handle } }
+    }
+}
+
+// ── shader::Program impl ──────────────────────────────────────────────────
+
+impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for ViewportPane<'a> {
+    type State = CameraState;
+    type Primitive = Primitive;
+
+    fn draw(
+        &self,
+        state: &Self::State,
+        _cursor: mouse::Cursor,
+        bounds: Rectangle,
+    ) -> Self::Primitive {
+        match &self.mode {
+            ViewportPaneMode::Model => {
+                self.scene.build_primitive(state.hover_region, bounds)
+            }
+            ViewportPaneMode::PaperSheet => {
+                self.scene.build_paper_sheet_primitive(state.hover_region, bounds)
+            }
+            ViewportPaneMode::Paper { handle } => {
+                self.scene.build_viewport_primitive(*handle, state.hover_region, bounds)
+            }
+        }
+    }
+
+    fn update(
+        &self,
+        state: &mut Self::State,
+        event: &Event,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) -> Option<iced::widget::Action<Msg>> {
+        // ViewCube hover only makes sense in the full model-space view.
+        if matches!(self.mode, ViewportPaneMode::Model | ViewportPaneMode::PaperSheet) {
+            self.scene.update_viewcube_state(state, bounds, cursor);
+        }
+        let _ = event;
+        None
+    }
+
+    fn mouse_interaction(
+        &self,
+        state: &Self::State,
+        _b: Rectangle,
+        _c: mouse::Cursor,
+    ) -> mouse::Interaction {
+        if matches!(self.mode, ViewportPaneMode::Model | ViewportPaneMode::PaperSheet) {
+            self.scene.viewcube_mouse_interaction(state)
+        } else {
+            mouse::Interaction::default()
+        }
+    }
+}