feat(paper): render-mode picker + ViewCube on active viewport

Paper-space'de bir viewport'a çift tıklanıp aktif hale gelince, model
space'deki gibi sol üst köşede render-mode dropdown'ı ve sağ üst köşede
ViewCube gösteriliyor.

- render_mode_picker helper'a çıkarıldı; model space ve aktif paper
  viewport ortak kullanıyor (kod tekrarı yok)
- SetRenderMode artık viewport-aware: aktif viewport varsa o viewport
  entity'sinin render_mode'unu, yoksa model-layout tab stilini günceller
  (Scene::active_viewport_render_mode getter/setter)
- build_viewport_primitive'e show_viewcube parametresi; aktif viewport
  primitive'i ViewCube gizmo'sunu çiziyor
- PaperViewportPane shader::Program artık Message'a özelleşti ve sol
  tık ViewCube hit-test'ini bounds-relative yapıp ViewCubeSnap üretiyor

Bilinen sorun: picker/viewcube doğru konumda görünüyor ancak tıklamalar
üstteki viewport_mouse mouse_area katmanına gidiyor; katman sırası bir
sonraki adımda düzeltilecek.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/mod.rs

@@ -2199,6 +2199,36 @@ impl Scene {
         }
     }
 
+    /// Render mode of the active paper-space viewport, or `None` when no
+    /// viewport is active (PSPACE / model layout).
+    pub fn active_viewport_render_mode(
+        &self,
+    ) -> Option<acadrust::entities::ViewportRenderMode> {
+        let h = self.active_viewport?;
+        match self.document.get_entity(h) {
+            Some(acadrust::EntityType::Viewport(vp)) => Some(vp.render_mode),
+            _ => None,
+        }
+    }
+
+    /// Set the active paper-space viewport's render mode. Returns `true`
+    /// when a viewport was active and updated; `false` (no-op) otherwise,
+    /// so the caller can fall back to the model-layout render mode.
+    pub fn set_active_viewport_render_mode(
+        &mut self,
+        mode: acadrust::entities::ViewportRenderMode,
+    ) -> bool {
+        let Some(h) = self.active_viewport else {
+            return false;
+        };
+        if let Some(acadrust::EntityType::Viewport(vp)) = self.document.get_entity_mut(h) {
+            vp.render_mode = mode;
+            true
+        } else {
+            false
+        }
+    }
+
     /// View-rotation matrix for the active viewport (MSPACE), or the
     /// paper-space camera's matrix when not in MSPACE.
     /// Used by ViewCube hit-testing so clicks map to the correct camera.

src/scene/render.rs

@@ -512,6 +512,7 @@ impl Scene {
         vp_handle: Handle,
         hover_region: Option<usize>,
         bounds: Rectangle,
+        show_viewcube: bool,
     ) -> Primitive {
         let render_mode = match self.document.get_entity(vp_handle) {
             Some(EntityType::Viewport(vp)) => vp.render_mode,
@@ -551,7 +552,7 @@ impl Scene {
             cam_rotation: cam.view_rotation_mat(),
             hover_region,
             bg_color: self.bg_color,
-            show_viewcube: false,
+            show_viewcube,
             fill_mode: self.document.header.fill_mode,
             view_wireframe,
             mesh_fill: flags.mesh_fill,
@@ -570,8 +571,10 @@ impl Scene {
         hover_region: Option<usize>,
         bounds: Rectangle,
     ) -> PaperViewportPrimitive {
+        // The active (double-clicked) viewport shows the ViewCube gizmo,
+        // mirroring the model-space view.
         PaperViewportPrimitive(
-            self.build_viewport_primitive(vp_handle, hover_region, bounds),
+            self.build_viewport_primitive(vp_handle, hover_region, bounds, true),
         )
     }
 

src/scene/viewport_pane.rs

@@ -1,5 +1,6 @@
 use super::render::{CameraState, PaperViewportPrimitive, Primitive};
-use super::Scene;
+use super::{Scene, VIEWCUBE_PX};
+use crate::app::Message;
 use acadrust::Handle;
 use iced::widget::shader;
 use iced::{mouse, Event, Rectangle};
@@ -70,7 +71,7 @@ impl<'a> PaperViewportPane<'a> {
     }
 }
 
-impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for PaperViewportPane<'a> {
+impl<'a> shader::Program<Message> for PaperViewportPane<'a> {
     type State = CameraState;
     type Primitive = PaperViewportPrimitive;
 
@@ -90,9 +91,30 @@ impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for PaperViewportPan
         event: &Event,
         bounds: Rectangle,
         cursor: mouse::Cursor,
-    ) -> Option<iced::widget::Action<Msg>> {
+    ) -> Option<iced::widget::Action<Message>> {
         self.scene.update_viewcube_state(state, bounds, cursor);
-        let _ = event;
+        // A left click on the ViewCube snaps the viewport's view. The
+        // hit-test runs in bounds-relative coordinates so it matches the
+        // gizmo drawn in this viewport's top-right corner. Clicks outside
+        // the cube return None, so they fall through to the paper sheet
+        // for normal selection / drawing.
+        if let Event::Mouse(mouse::Event::ButtonPressed(mouse::Button::Left)) = event {
+            if let Some(pos) = cursor.position_in(bounds) {
+                let rot = self.scene.active_view_rotation_mat();
+                if let Some(region) = super::hit_test(
+                    pos.x,
+                    pos.y,
+                    bounds.width,
+                    bounds.height,
+                    rot,
+                    VIEWCUBE_PX,
+                ) {
+                    return Some(iced::widget::Action::publish(Message::ViewCubeSnap(
+                        region,
+                    )));
+                }
+            }
+        }
         None
     }
 
@@ -127,7 +149,7 @@ impl<'a, Msg: std::fmt::Debug + Clone> shader::Program<Msg> for ViewportPane<'a>
             ),
             ViewportPaneMode::Paper { handle } => {
                 self.scene
-                    .build_viewport_primitive(*handle, state.hover_region, bounds)
+                    .build_viewport_primitive(*handle, state.hover_region, bounds, false)
             }
         }
     }