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/pad.rs

@@ -23,7 +23,7 @@ impl Widget for Pad {
     }
 
     /// The padding is an offset from each edge, so a longer box pads the same
-    /// amount and the child takes the rest.
+    /// amount and the child takes what is left over.
     fn on_resize(&self, _: Axis) -> OnResize {
         OnResize::Scale
     }