feat(ui): linear-gradient backgrounds, object-fit and aspect-ratio

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

changes/defaults-views-templates.md

@@ -98,6 +98,8 @@ What a game's screens found missing, now in `ludic.ui`:
   pixels (`ui_nine_cuts`), so a small key cap has no seam;
 - `scroll-top="{px}"` holds a scroll box at an offset, with `on-scroll` when the player moves it;
   `ui_scroll_set(id, px)` moves one once;
+- `linear-gradient(...)` backgrounds; `aspect-ratio`; `object-fit` for pictures (the renderer's
+  `image_w` / `image_h`) and `ui_object_fit` for natives;
 - a transition lands exactly on its end value (it had stopped a rounding error short of it, at every
   frame rate).