Read a child's answer in the asker's frame, and drop the root move entry

A widget reports a fraction of the box it was given. Span added that
fraction straight into a cursor that counts fractions of the row, and Pad
summed its padding onto it, both right only while the offer had the
parent's whole extent -- which a span's does not after a relative child.
DrawResult::size and known_len now compose the answer through the offer's
length, so a container reads lengths of its own box.

That exposed placed_box scaling a fractional answer against a box the
parent had already chosen from it, halving a nested span twice. The
near-edge alignment override becomes per-axis `decided` flags: a box the
parent chose from the answer is the answer, and is not placed again.
Span decides the row axis; Scroll and Stack's sizing child decide both.
Alignment is always the widget's own property now.

The window is no longer a move entry. Chains bottom out in MoveIdx::NONE
and the window is applied where a fraction becomes pixels, in to_px on the
CPU and by the uniform in the shader, which now snaps the summed coordinate
since a floor does not distribute over a sum. A resize rewrites no entry.

Verified: view, minimal, random, tabs and text render byte-identical at
1920x1200 against 5f16617, a live resize to 1280x800 is identical to a
cold render, and the 100-seed oracle, all fifteen shrinker cases at 400
seeds of depth 5, and 1000 seeds of depth 6 pass.

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

src/widget/position/stack.rs

@@ -35,7 +35,7 @@ impl Widget for Stack {
             // child is handed a box that owes nothing to its own answer, and
             // where it sits in one bigger than itself is its own business.
             match sizing == Some(i) {
-                true => painter.widget_aligned(child, region, RegionAlign::NEAR),
+                true => painter.widget_decided(child, region, [true; 2]),
                 false => painter.widget_within(child, region),
             };
         }