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>

tests/determinism.rs

@@ -24,15 +24,15 @@ impl Widget for BranchesOnMeasurement {
         let mut top = UiRegion::FULL;
         top.y.end = top.y.start.offset(40.0);
         let measured = painter.place(&self.probe, top).len(Axis::X);
-        let px = measured.apply_rest().to_px(painter.px_len(Axis::X));
+        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
 
-        let mut rest = UiRegion::FULL;
-        rest.y.start = rest.y.start.offset(40.0);
+        let mut below = UiRegion::FULL;
+        below.y.start = below.y.start.offset(40.0);
         match px > self.threshold {
-            true => painter.place(&self.wide, rest),
-            false => painter.place(&self.narrow, rest),
+            true => painter.place(&self.wide, below),
+            false => painter.place(&self.narrow, below),
         };
-        Size::REST
+        Size::LEFTOVER
     }
 }