Split large files: ribbon widgets and scene render/selection

- ribbon.rs → ribbon/mod.rs + ribbon/widgets.rs
  Extracted all rendering helpers, button styles, colour constants,
  layout constants and dropdown position calculators into widgets.rs.

- scene/mod.rs → scene/mod.rs + scene/selection.rs + scene/render.rs
  Moved SelectionState into selection.rs; moved CameraState, Primitive,
  shader::Program/Primitive impls, render_style and resolve_pattern
  into render.rs, reducing mod.rs from 1323 to ~1030 lines.

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

src/scene/selection.rs

@@ -0,0 +1,30 @@
+use iced::time::Instant;
+use iced::Point;
+
+/// Mouse / selection interaction state for the viewport.
+#[derive(Clone, Default)]
+pub struct SelectionState {
+    pub vp_size: (f32, f32),
+    pub box_anchor: Option<Point>,
+    pub box_current: Option<Point>,
+    pub box_last: Option<(Point, Point)>,
+    pub box_crossing: bool,
+    pub box_last_crossing: bool,
+    pub poly_active: bool,
+    pub poly_points: Vec<Point>,
+    pub poly_crossing: bool,
+    pub poly_last_crossing: bool,
+    pub context_menu: Option<Point>,
+    pub last_move_pos: Option<Point>,
+    pub left_down: bool,
+    pub left_press_pos: Option<Point>,
+    pub left_press_time: Option<Instant>,
+    pub left_dragging: bool,
+    pub right_down: bool,
+    pub right_press_pos: Option<Point>,
+    pub right_dragging: bool,
+    pub right_last_pos: Option<Point>,
+    pub middle_down: bool,
+    pub middle_last_pos: Option<Point>,
+    pub middle_last_press_time: Option<Instant>,
+}