Give a child a part of the container's extent rather than its raw box

`Pad` and `Stack` read `Painter::placement` to put their children inside
their own drawing, and reading it is what says the drawing holds for that
placement alone. So a pad or a stack anywhere in a row was drawn again --
with its whole subtree -- the moment an earlier sibling changed length,
however little else had moved.

`widget_within` now takes a `DrawRegion`, and `DrawRegion::Extent(part)`
gives the child a part of the extent without reading it. What is retained
is the part rather than the box it resolved to, so moving the extent
re-places the child through the same rule instead of redrawing the parent:
`inherited_children` becomes `extent_children`, carrying `Inherit` for the
wrapper case `Painter::widget` already had and `Within(part)` for the new
one.

The dependency that goes up is a range on the container's extent rather
than on its frame, since only the part's *length* reaches the child and
where the part sits is re-placed. A declared length is unchanged: it is a
length of the frame wherever the box it sits in came from. What still pins
the placement is a report with a fraction in it -- the same fraction of a
different extent is a different length -- and that pin is on the answer,
which `extent_frames_keep_fractional_reports_and_numeric_dependencies_valid`
fails without.

Three tests from the first attempt at this come with it, and the
diagnostics rig now says which of the three contracts refused a reuse,
which is what found the above.

Measured, seed 1 at depth 8, median frame: `many` 0.667 -> 0.613 ms and
`resize` 48 -> 32 us; seed 13's `many` 6.35 -> 5.15 ms. Green: fmt, clippy,
109 suite and 20 core tests, the oracle at 100 seeds, the shrinker at 400
trees of depth 5, 1000 seeds at depth 6, and 2000 seeds at depth 4 over all
fifteen cases. The five reference renders are byte-identical to `0e107f0`
on Venus, as are `tabs` resized to 900x1200 and `random` to 1280x800
against cold renders there.

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

src/widget/position/stack.rs

@@ -13,7 +13,6 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        let placement = painter.placement();
         // Whichever child sizes the stack keeps the stack's whole region as
         // its own -- the stack is the length that child asked for, so taking
         // the fraction of the stack's box again would take it twice -- and is
@@ -24,13 +23,7 @@ impl Widget for Stack {
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter
-                    .widget_at(
-                        child,
-                        UiRegion::FULL,
-                        [Some(placement.x), Some(placement.y)],
-                    )
-                    .size()
+                painter.widget(child).size()
             }
             None => Size::LEFTOVER,
         };
@@ -42,7 +35,7 @@ impl Widget for Stack {
             // Every other child has the stack's own box for its region, since
             // the stack is what contains it, and where it sits in one bigger
             // than itself is its own business.
-            painter.widget_within(child, placement);
+            painter.widget_within(child, DrawRegion::Extent(UiRegion::FULL));
         }
         size
     }