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/align.rs

@@ -14,11 +14,17 @@ impl Widget for Aligned {
                 .map(|(x, y)| Size { x, y }),
             (Some(_), None) => painter
                 .known_len(&self.inner, Axis::X, UiRegion::FULL)
-                .map(|x| Size { x, y: Len::REST }),
+                .map(|x| Size {
+                    x,
+                    y: Len::LEFTOVER,
+                }),
             (None, Some(_)) => painter
                 .known_len(&self.inner, Axis::Y, UiRegion::FULL)
-                .map(|y| Size { x: Len::REST, y }),
-            (None, None) => Some(Size::REST),
+                .map(|y| Size {
+                    x: Len::LEFTOVER,
+                    y,
+                }),
+            (None, None) => Some(Size::LEFTOVER),
         };
         // Drawn where it may be too big only when the aligned axes are not
         // already known, then given its aligned box once its size is known.
@@ -26,8 +32,8 @@ impl Widget for Aligned {
         let size = known.unwrap_or_else(|| painter.place(&self.inner, UiRegion::FULL).size());
         let region = match self.align.tuple() {
             (Some(x), Some(y)) => size.to_uivec2().align(RegionAlign { x, y }),
-            (Some(x), None) => UiRegion::new(size.x.apply_rest().align(x), UiSpan::FULL),
-            (None, Some(y)) => UiRegion::new(UiSpan::FULL, size.y.apply_rest().align(y)),
+            (Some(x), None) => UiRegion::new(size.x.apply_leftover().align(x), UiSpan::FULL),
+            (None, Some(y)) => UiRegion::new(UiSpan::FULL, size.y.apply_leftover().align(y)),
             (None, None) => UiRegion::FULL,
         };
         let placed = painter.place(&self.inner, region).size();