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/pipeline/mod.rs

@@ -19,10 +19,10 @@ pub use uniforms::Uniforms;
 pub use viewcube::ViewCubePipeline;
 pub use wire_gpu::WireGpu;
 
-use crate::scene::hatch_model::HatchModel;
-use crate::scene::image_model::ImageModel;
-use crate::scene::mesh_model::MeshLodSet;
-use crate::scene::wire_model::WireModel;
+use crate::scene::model::hatch_model::HatchModel;
+use crate::scene::model::image_model::ImageModel;
+use crate::scene::model::mesh_model::MeshLodSet;
+use crate::scene::model::wire_model::WireModel;
 
 /// MSAA sample count for the main drawing pipelines.
 const MSAA_SAMPLES: u32 = 4;