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/mtext_editor.rs

@@ -323,8 +323,8 @@ pub fn visible_spans(raw: &str) -> Vec<(usize, usize)> {
 
 // ── App-side editor driver ──────────────────────────────────────────────────
 
-use crate::scene::tessellate;
-use crate::scene::wire_model::WireModel;
+use crate::scene::convert::tessellate;
+use crate::scene::model::wire_model::WireModel;
 use iced::widget::text_editor::{Action, Edit};
 use std::sync::Arc;