Compose a box down its chain once, not once a level

Bryan's call, 2026-09-16, for correctness. `Moves` walked the move chain in
`Len`, so every level's four multiplies landed back on the grid before the
next started and the residue grew with the depth of the tree. `WideLen`
carries a length through the walk on a grid twenty-four bits of a box and
twenty-two of a pixel finer, and rounds once at the end.

What it buys, measured rather than argued: `Holds::through`'s allowance for
the two routes to a length drops from three half steps to two, and the whole
of a box now maps back to a range one step wide rather than one step per
level of nesting. One half step further is arithmetically available -- the
`Holds` assertion is quiet there and the whole-box case becomes an exact
identity -- and it is **not taken**, because shrinker seed 220 then lays out
differently warm than cold. Too narrow is meant to cost a redraw and no more;
there it re-breaks a wrapping text, whose reported width moves a `Branch`
onto its other subtree. That is the unsettled-text family, and closing it is
what would let this go lower. The note is in `through`.

`Moves` now answers three questions instead of one, and they are different
questions: `size_of` for how long a box is, which is what reads a box in
pixels; `compose` for where both of its ends are, which is what compares two
boxes; and `resolve`, unchanged, for the `Len` walk the vertex shader does
again in floats. A length composes on its own in two multiplies a level
rather than four, since where the parent sits falls out of the difference --
which is most of why this is not slower.

Measured on the fixed-shape fixture, seed 1 depth 8, 500 frames of `many`,
medians of 25 runs with all twenty-five work counters identical between the
two: 1,880M instructions and 755M cycles against 1,908M and 760M. So it is
free, and a little better on instructions. Three things were tried on the way
and two kept: composing the length alone rather than both ends (-111M
instructions), taking the pixel term's fraction on the ordinary grid so it
stays in an `i64` (-2M instructions, -8M cycles), and skipping a parent that
spans its own box, which **cost** 18M instructions and is not here -- the
same verdict a short-circuit got in `UiSpan::within`.

Checked: fmt, clippy, 83 suite tests, 17 core unit tests, the release oracle
at 100 seeds and at 1000 seeds of depth 6, all fifteen shrinker cases at 400
seeds of depth 5, and `tabs`, `view`, `minimal`, `text` and `random`
byte-identical at 1920x1200 against `d21a215`.

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

core/src/ui/holds.rs

@@ -55,28 +55,40 @@ impl Holds {
         if rel == 0 {
             return Self::ANY;
         }
-        // In half steps, and no more than the arithmetic needs: too wide a
-        // range admits reusing a drawing where it does not hold, and too
-        // narrow a one leaves out the box a drawing was made in, which the
-        // `Holds` assertion in `draw_at` catches. `ROUTES` is at that floor
-        // -- two half steps fires it -- and tightening both ends moved not
-        // one of the rig's work counters, so the slack is not buying reuse.
+        // In half steps, and both at the floor: one less on either and the
+        // `Holds` assertion in `draw_at` fires, because the range stops
+        // containing the box a drawing was made in. Wider is the unsound
+        // side -- it admits reusing a drawing where it does not hold -- and
+        // neither end buys any reuse, since tightening them moves none of
+        // the rig's work counters.
         //
         // `ROUTES` covers a box composed down the chain against the same box
-        // measured against the window: two routes to one number, each
-        // rounding where the other does not. `way_in` covers the multiply
-        // this inverts, which drops a step and only ever downward, so it
-        // belongs at the top of the range alone -- and the whole of a box has
-        // no multiply in it, however many pixels were added to it, since
-        // multiplying by one is exact and taking the pixels off again is too.
-        // Allowing for it there anyway compounded, a step a level down a
-        // chain of widgets each taking the whole of its parent.
+        // measured against the window. It was three half steps while
+        // composing rounded four multiplies a level; `Moves::compose` now
+        // rounds the whole walk once, which took one off. One more is
+        // arithmetically available -- the `Holds` assertion is quiet at one
+        // half step, and the whole-of-a-box case maps back to itself exactly
+        // -- and it is **not** taken, because a range that tight makes
+        // shrinker seed 220 lay out differently warm than cold. Too narrow
+        // is supposed to cost only a redraw; there it re-breaks a wrapping
+        // text, whose reported width then moves a `Branch` onto its other
+        // subtree. That is the unsettled-text family rather than a rounding
+        // question, and closing it is what would let this go lower.
+        //
+        // `way_in` covers the multiply this inverts, which drops a step and
+        // only ever downward, so it belongs at the top of the range alone.
+        // It is irreducible for the same reason a floor is not invertible:
+        // many boxes give one length. The whole of a box has no multiply in
+        // it, however many pixels were added to it, since multiplying by one
+        // is exact and taking the pixels off again is too -- allowing for it
+        // there anyway compounded, a step a level down a chain of widgets
+        // each taking the whole of its parent.
         //
         // Shifted by half of what a `Rel` counts in, to divide by the
         // fraction: exact until the division takes it back to the grid.
-        const ROUTES: i64 = 3;
         let px = len.px.raw() as i64;
         let half_rel = REL_SHIFT - 1;
+        const ROUTES: i64 = 2;
         let way_in = match rel == Rel::ONE.raw() as i64 {
             true => 0,
             false => 2,
@@ -136,11 +148,10 @@ mod tests {
     }
 
     /// A widget handed the whole of its parent's box, with or without pixels
-    /// taken off it, brings no multiply of its own: only the two routes to
-    /// the same length are left to allow for, and not a rounding that did
-    /// not happen. Widening for it as well grew the interval a level at a
-    /// time down a chain of them. Three half steps come back as one whole
-    /// one, since dividing by a whole box is dividing by one.
+    /// taken off it, brings no multiply of its own, so it allows for the two
+    /// routes and nothing else -- one step, where it was one a level of
+    /// nesting before `Moves::compose`. The identity is what the arithmetic
+    /// would allow; see `through` for why it is not taken.
     #[test]
     fn the_whole_of_a_box_widens_by_the_routes_alone() {
         let at = Px::from_int(956);