Take two roundings out of where a box comes from

Traced what was left of the warm-against-cold difference after fixed point.
It is not accumulation and not one place: it is the same box reached two
ways, and each way rounds where the other does not.

`Scroll` was writing a box it had been given back out as its own length in
pixels. That is the same box in another form, and centring a part in `rel 1`
lands a step from centring it in `px 900`, because halving a difference is
not halving each part of it. Content that fills the viewport and has not been
scrolled is now handed back as it came, which makes the shrinker's `repaint`
and `resize-repaint` cases agree exactly rather than within a step.

`Span` placed each child a step from where the last one ended, so the
rounding of every share was carried along the row. A position is now the
fixed parts before it -- a sum, exact -- plus one rounded share of the room.
Measured: two hundred equal shares of a 1000 px row ended at 999.999 and now
end at 1000, and `tests/cases/layout.rs` pins it at 2, 3, 7, 64 and 200.

What is left is a step per level of nesting between the two ways, which is
what the fuzzers now allow: four of the five shrinker cases pass at one step
and the fifth is five spans deep. Closing it needs one way of asking where a
box is, which is a bigger change than this.

Checked: fmt, clippy, 103 tests, all five shrinker cases at 300 seeds, 100
generated seeds, five examples byte-identical at 1920x1200.

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

tests/shrink.rs

@@ -51,9 +51,9 @@ const WORDS: &[&str] = &[
 const ONE_LINE: &str = "one line, overflowing whatever it is given";
 
 const OUTER: (f32, f32) = (1920.0, 1200.0);
-/// Steps of the grid two ways of reaching a box may differ by. See
-/// `docs/HANDOFF.md`'s "Fixed point" in `ai-app-2` for where the last of
-/// them is.
+/// Steps of the grid two ways of reaching a box may differ by: one per level
+/// of nesting between them, and these trees are five deep. See
+/// `docs/HANDOFF.md`'s "Fixed point" in `ai-app-2` for what is left.
 const AGREE_STEPS: i32 = 2;
 const INNER: (f32, f32) = (640.0, 900.0);