Give a slot to the children a container places, and nothing else

A widget's region is now held in the coordinates of the slot it draws in
rather than the window's, and `Painter::place` is how a container asks for a
slot: it draws a child it decides the box of and may decide again. Everything
under that slot is a fraction of its box, so placing the child a second time
is one entry to write whether it moved or changed length. A child drawn any
other way has no slot and shares its nearest ancestor's.

That is what keeps the chain short. `chain_cost` measured depth as the cost
-- free to 8, +42.6% at 16 -- and a slot per widget put a transcript's glyphs
past that for nothing, since almost every slot was zero. `Span`, `Aligned`
and `Scroll` are the containers that re-place a child after drawing it, and
`tests/layout.rs` pins that four widgets between a span and a leaf leave the
leaf's chain one deep.

`UiRegion::stretch`, `UiRegion::stretchable` and `UiScalar::stretch` are
gone. Nothing is inverted any more: a box that changed length is written to
its slot, and the descendants recompose against it in the shader. That also
retires the case the guard existed for, where a fixed length has no fraction
to recover -- `tests/layout.rs` now stretches a 40-tall row on its other
axis, which `stretchable` refused outright.

What still walks the CPU is deciding who must draw again, which no chain can
answer: `mark_resized` descends from the widget whose box changed and marks
anything whose own box changed length and whose drawing reads it. A part of
a box with no relative extent on an axis is a fixed length, and composing
into it leaves none either, so the walk stops where a length did not change
-- an 80-wide child in a widened row is not redrawn though it says `Redraw`.

`Span`, `Pad`, `Stack`, `Offset`, `Aligned`, `SetSize` and `LayerOffset` say
`Scale`: each places in fractions and offsets of its own box and none reads
the box's pixel length. `Scroll` and `MaxSize` do read pixels and stay
`Redraw`.

45 tests pass, five of them new. Render verification comes after the CPU
side, per the owner.

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

core/src/orientation/pos.rs

@@ -202,17 +202,6 @@ impl UiScalar {
         }
     }
 
-    /// `within` undone against `from` and redone against `to`, for one axis.
-    /// `from`'s relative extent is the denominator, so it must not be zero.
-    fn stretch(&self, from: &UiSpan, to: &UiSpan) -> Self {
-        let frac = (self.rel - from.start.rel) / (from.end.rel - from.start.rel);
-        Self {
-            rel: frac.lerp(to.start.rel, to.end.rel),
-            abs: self.abs - frac.lerp(from.start.abs, from.end.abs)
-                + frac.lerp(to.start.abs, to.end.abs),
-        }
-    }
-
     pub fn within_len(&self, len: UiScalar) -> Self {
         self.within(&UiSpan {
             start: UiScalar::ZERO,
@@ -391,30 +380,6 @@ impl UiRegion {
             },
         }
     }
-
-    /// Whether a stretch out of this box can be expressed. Each part inside a
-    /// box is held as a fraction of it, and a fixed length has no fraction to
-    /// hold one by -- every part of it is just an offset from its start.
-    pub fn stretchable(&self) -> bool {
-        self.x.start.rel != self.x.end.rel && self.y.start.rel != self.y.end.rel
-    }
-
-    /// Re-expresses a region inside `from` as the same fractions of `to`.
-    /// `from` must be `stretchable`; a translation is `shift` instead, which
-    /// needs no fractions and works out of any box.
-    pub fn stretch(&self, from: &UiRegion, to: &UiRegion) -> UiRegion {
-        debug_assert!(from.stretchable(), "a fixed length has no fraction");
-        UiRegion {
-            x: UiSpan {
-                start: self.x.start.stretch(&from.x, &to.x),
-                end: self.x.end.stretch(&from.x, &to.x),
-            },
-            y: UiSpan {
-                start: self.y.start.stretch(&from.y, &to.y),
-                end: self.y.end.stretch(&from.y, &to.y),
-            },
-        }
-    }
 }
 
 impl Display for UiRegion {