Say what a child gets of a container's box in that box's own lengths

Three things the measurements asked for, all about how much a box that came
from an answer costs.

**A part in the box's own coordinates.** Saying "less eleven pixels at the
end" in frame lengths from the box's start means reading how long the box is,
and a container whose box is its own answer then depends on its own answer:
`Pad` drew sixty-four times in one resize frame at seed 13, chasing its own
width. `Part::Of` says the same thing as a part of the box, which composes
without a length -- pixels are pixels wherever the box lands -- and what a
child under it holds for maps back through that part onto the container's
own box rather than onto the frame.

**One axis of the box at a time.** `extent_len` pinned both axes, so a span
dividing one of them held for one length of the other as well, and a resize
broke every span whose cross-axis answer moved.

**No lazy placement.** Leaving a child's answer to be placed at the end of
the parent's draw, rather than as the child answers, was meant to save a
recomposition. It costs one instead: the drawing is put in the part first
and in the answer's box after, and where it does not hold for both that is
two drawings rather than one. Seed 1 at depth 8 went from 391 widget draws
on a resize to 29 with it gone. The test that pinned three draws for a
numeric leaf in a span goes with it.

Seed 1 at depth 8, widget draws / distinct widgets / update, against #18's
head and against the commit this branch started from:

| phase   | e44dea3      | 34cafb6      | here          |
| ---     | ---          | ---          | ---           |
| cold    | 369/261/10.6 | 463/274/13.3 | 516/288/12.0  |
| repaint | 1            | 1            | 1             |
| many    | 157/95/0.33  | 263/108/0.59 | 187/119/0.52  |
| size    | 16/12/0.018  | 3/3          | 3/3/0.010     |
| scroll  | 2/0.002      | 1            | 1/0.004       |
| resize  | 13/13/0.019  | 22/15/0.032  | 24/76/0.090   |

Seed 13 at depth 8 is where the protocol still costs: `many` 1091 draws
against #18's 524, and `resize` 2215 against a frame #18 does not draw at
all. Both are the same shape -- an answer measured in one box and drawn in
another -- and the handoff says where that comes from.

Checked: fmt, clippy with -D warnings, 108 suite tests, 20 core tests, the
11 generated cases, and the shrinker at 400 trees of depth 5, which fails
seeds 2 (repaint) and 108 (reorder).

src/widget/position/stack.rs

@@ -23,7 +23,7 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, UiRegion::FULL, [Place::Fill(None); 2])
+                    .widget_at(child, UiRegion::FULL, [Place::Fill(Part::All); 2])
                     .size()
             }
             None => Size::LEFTOVER,