Rename rest to leftover

The length kind that asks for a part of what is left once the fixed
lengths are taken is called leftover: Len::leftover(2), Len::LEFTOVER,
Size::LEFTOVER, Len::leftover the field, and apply_leftover. It says
what it is where "rest" reads as "the remainder of the list" as often as
"the remaining space", and every agent who has touched this has reached
for a third word for it.

Locals called rest that meant a region or a widget are renamed with it,
since the word now names something else.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/widget/position/scroll.rs

@@ -21,7 +21,7 @@ impl Widget for Scroll {
         let child = measured.then(|| painter.place(&self.inner, UiRegion::FULL).size());
         let content_len = known_len
             .unwrap_or_else(|| child.unwrap().axis(self.axis))
-            .apply_rest()
+            .apply_leftover()
             .within_len(container_len)
             .to_px(output_len);
         self.container_len = container_len.to_px(output_len);