Measure a dirty widget where its parent asked, not in a box its answer decided

A local redraw drew a dirty widget in the box it was placed in. When a reader
decided that box from the widget's own answer -- an aligned span sized to its
children, a text at the tail of a row, a scroll's content -- the old answer is a
fixed point of measuring there whatever the content now says, so the layout had
two stable answers and which one it reached depended on the tree's history.
`tests/unsettled.rs` has the two shrunk cases: the four-widget aligned span,
and a scroll placing a pass-through `SetSize` in a box the content decided,
where the span under it was placed once and nothing at its own edge said so.

`ActiveData::offered_px` keeps the pixel size of the box the parent first asked
about the child in, whether through `known_len` or a first `place`, beside `px`,
the box it drew against. A dirty widget whose size reads an axis on which some
reader up its chain gave what it read a box other than the one it asked in is
not drawn locally: the chain is marked and the parent of the highest such
placement draws, since above it every box is a constraint rather than an
answer. The walk goes up the whole reader chain because a pass-through hands a
derived box down unchanged.

`Scroll` read its box's length for the clamp through `px_len`, which records
the reported size as depending on it, and it does not: its size is its
content's. That made every scroll tick a size question asked in a derived box,
at 34x the instructions. `Painter::px_len_for_draw` is the read that records
nothing. Instructions per frame on the depth-8 rig against the previous head:
`many` at 32 dirty 0.66M to 0.74M, at 130 dirty 27.7M to 26.5M, `resize` 15.8M
to 15.0M, `scroll`, `repaint` and `size` unchanged. The shrinking fuzzer passes
200 trees at depth 7 in all four cases, the hundred-seed sweep passes, and the
five reference renders and the resize render are byte-identical.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/widget/position/scroll.rs

@@ -12,7 +12,8 @@ pub struct Scroll {
 impl Widget for Scroll {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let output_len = painter.output_len(self.axis);
-        let container_len = UiScalar::px(painter.px_len(self.axis));
+        // Its size is its content's, whatever box that is scrolled within.
+        let container_len = UiScalar::px(painter.px_len_for_draw(self.axis));
         // Draw in the whole container only when its scrolling-axis length is
         // not already known, then place it at the scrolled offset.
         let known_len = painter.known_len(&self.inner, self.axis, UiRegion::FULL);