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/app/mod.rs

@@ -50,7 +50,7 @@ pub struct GripPopup {
     pub handle: acadrust::Handle,
     pub grip_id: usize,
     pub anchor: iced::Point,
-    pub items: Vec<crate::scene::object::GripMenuItem>,
+    pub items: Vec<crate::scene::model::object::GripMenuItem>,
     pub selected: usize,
 }
 
@@ -62,7 +62,7 @@ pub struct GripPopup {
 pub struct GripPendingValue {
     pub handle: acadrust::Handle,
     pub grip_id: usize,
-    pub action: crate::scene::object::GripMenuAction,
+    pub action: crate::scene::model::object::GripMenuAction,
     pub label: &'static str,
 }