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 <[email protected]>

src/ui/textstyle.rs

@@ -256,7 +256,7 @@ pub fn view_window<'a>(v: TextStyleView<'a>) -> Element<'a, Message> {
     });
 
     // ── System TrueType fonts (read from the user's machine via fontdb) ──
-    let ttf_items: Vec<Element<'_, Message>> = crate::scene::sysfont::families()
+    let ttf_items: Vec<Element<'_, Message>> = crate::scene::text::sysfont::families()
         .iter()
         .map(|fam| {
             let is_sel = ttf_buf.eq_ignore_ascii_case(fam);