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/view.rs

@@ -3,8 +3,8 @@ use super::document::{DynComponent, DynFieldEntry};
 use super::helpers::grid_plane_from_camera;
 use super::history::history_dropdown_labels;
 use super::{Message, OpenCADStudio};
-use crate::scene::grip::{grips_to_screen, grips_to_screen_paper};
-use crate::scene::viewport_pane::ViewportPane;
+use crate::scene::pick::grip::{grips_to_screen, grips_to_screen_paper};
+use crate::scene::view::viewport_pane::ViewportPane;
 use crate::scene::{VIEWCUBE_DRAW_PX, VIEWCUBE_PAD};
 use crate::ui::overlay;
 use iced::widget::{