refactor(scene): group modules into role-based subfolders

Move the 30 flat modules under src/scene/ into six folders by role:
convert (entity→geometry), text (fonts/shaping), model (render models),
pick (hit-test/selection/grips/xclip), view (camera/transform/render),
cache (block + property caches). pipeline/ is unchanged.

Full path migration — all `scene::X` references become
`scene::<group>::X`, no compatibility re-exports. render.rs items that
were `pub(super)` (parent was scene) become `pub(in crate::scene)` to
keep their original scene-wide visibility.

No behavior change; build and tests are green.

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

src/scene/view/viewport_pane.rs

@@ -0,0 +1,86 @@
+//! Unified GPU shader widget for both model and paper layouts.
+//!
+//! There is one `shader::Program` for the whole canvas. `Scene::build_viewports`
+//! decides what to render — model layout produces one viewport per tile
+//! (`active_viewports` iterates `model_tiles`), paper layout produces one per
+//! content viewport entity. Each one is drawn into its own scissor rect by a
+//! dedicated inner `Pipeline` (kept in the `MultiPipeline` outer).
+
+use super::render::{CameraState, Primitive};
+use crate::scene::Scene;
+use iced::widget::shader;
+use iced::{mouse, Event, Rectangle};
+
+// ── Widget struct ─────────────────────────────────────────────────────────
+
+pub struct ViewportPane<'a> {
+    pub scene: &'a Scene,
+    pub show_viewcube: bool,
+    /// Render mode applied to the Model layout's tiles. Paper-space content
+    /// viewports use the render mode stored on their own viewport entity.
+    pub render_mode: acadrust::entities::ViewportRenderMode,
+}
+
+impl<'a> ViewportPane<'a> {
+    pub fn model(
+        scene: &'a Scene,
+        show_viewcube: bool,
+        render_mode: acadrust::entities::ViewportRenderMode,
+    ) -> Self {
+        Self {
+            scene,
+            show_viewcube,
+            render_mode,
+        }
+    }
+}
+
+// ── 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 {
+        self.scene
+            .build_viewports(bounds, self.render_mode, state.hover_region)
+    }
+
+    fn update(
+        &self,
+        state: &mut Self::State,
+        event: &Event,
+        bounds: Rectangle,
+        cursor: mouse::Cursor,
+    ) -> Option<iced::widget::Action<Msg>> {
+        // ViewCube hover is also driven from the app-level CursorMoved /
+        // ViewportMove handlers (the cube hit-area overlay shadows this
+        // widget for those events). Keeping the call here gives a fallback
+        // path while the cursor is over the bare shader.
+        if self.show_viewcube {
+            self.scene.update_viewcube_state(state, bounds, cursor);
+        } else {
+            state.hover_region = None;
+        }
+        let _ = event;
+        None
+    }
+
+    fn mouse_interaction(
+        &self,
+        state: &Self::State,
+        _b: Rectangle,
+        _c: mouse::Cursor,
+    ) -> mouse::Interaction {
+        if self.show_viewcube {
+            self.scene.viewcube_mouse_interaction(state)
+        } else {
+            mouse::Interaction::default()
+        }
+    }
+}