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/selection.rs

@@ -1,32 +0,0 @@
-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>,
-    /// True while the context menu's Draw Order sub-items are expanded.
-    pub draworder_submenu: bool,
-    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>,
-}