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/model_ops.rs

@@ -9,7 +9,7 @@ use truck_modeling::Solid;
 
 use super::Message;
 use crate::modules::model::boolean_cmd::BoolOp;
-use crate::scene::model_solid::{self, Bool};
+use crate::scene::model::model_solid::{self, Bool};
 
 impl super::OpenCADStudio {
     /// Commit a Model-tab solid: add its acadrust entity to the document, then