Add the second shrunk case, and a trace rig for what box a text is drawn in

Six widgets from 905, and it fails in 0.06s: everything inside a declared
189x176 box is the same size whatever the output is, so a resize may not
reach any of it, and the text still comes out 3.92px narrower warm than
cold.

`tests/trace_unsettled.rs` says why, and it is not what it looked like. A
span measures a content-sized child in the space remaining, is told 167.41,
and then offers that back as the child's box -- so the text is re-broken at
exactly its own longest line, which is a knife edge: warm lands on four
lines and 163.49, cold stays on three and 167.41. Measuring an answer
against itself is unstable precisely at the fixed point.

`Painter::settle` -- move the child's slot, keep the drawing, never measure
again -- is the shape of the fix and does not work yet. In a span it breaks
five cases, because a container child may have laid its own children out as
fractions of the box it drew in, so moving it into a shorter one shrinks
them; reporting a length in pixels does not mean the drawing is positioned
in pixels. In `Aligned` alone it breaks two. Recorded rather than kept: the
condition wants to be something a widget declares, near `OnResize`, rather
than something its caller infers.

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

tests/trace_unsettled.rs

@@ -95,3 +95,61 @@ fn what_box_the_text_is_drawn_in() {
     }
     diag::clear_traced_widgets();
 }
+
+fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
+    let words = "Wrapping shapes one source into as many lines as the box leaves";
+    let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let aligned = Aligned {
+        inner: text.add_strong(&mut h.rsc),
+        align: Align {
+            x: Some(AxisAlign::Neg),
+            y: None,
+        },
+    }
+    .add(&mut h.rsc);
+    let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
+    let sized = SetSize {
+        inner: inner.add_strong(&mut h.rsc),
+        x: Some(Len::px(189.0)),
+        y: Some(Len::px(176.0)),
+    }
+    .add(&mut h.rsc);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        text.id(),
+        aligned.id(),
+        inner.id(),
+        sized.id(),
+        filler.id(),
+        root.id(),
+    ]
+}
+
+#[test]
+#[ignore = "a diagnostic, not a check"]
+fn what_box_the_fixed_text_is_drawn_in() {
+    diag::clear_traced_widgets();
+    let _ = diag::take();
+    let mut h = Harness::new((1920, 1200));
+    let ids = plant_fixed(&mut h);
+    let text = ids[0];
+    diag::trace_widget(text);
+    let _ = diag::take();
+
+    h.frame();
+    dump("first frame at 1920", &diag::take(), text);
+    h.resize((640, 900));
+    h.frame();
+    dump("after resize to 640", &diag::take(), text);
+
+    let mut cold = Harness::new((640, 900));
+    let cids = plant_fixed(&mut cold);
+    diag::clear_traced_widgets();
+    diag::trace_widget(cids[0]);
+    let _ = diag::take();
+    cold.frame();
+    dump("cold at 640", &diag::take(), cids[0]);
+    diag::clear_traced_widgets();
+}