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/render_state.rs

@@ -462,16 +462,14 @@ impl UiRenderState {
 
     /// The pixel size of a region held in `slot`'s coordinates.
     pub(super) fn px_of(&self, slot: MoveIdx, region: UiRegion) -> PxVec2 {
-        self.moves
-            .resolve(slot, region)
-            .size()
-            .to_px(self.output_size)
+        self.moves.size_of(slot, region).to_px(self.output_size)
     }
 
-    /// Where a region held in `slot`'s coordinates lands on screen, which is
-    /// the walk the vertex shader does.
+    /// Where a region held in `slot`'s coordinates lands on screen, to
+    /// compare one box against another. Both ends of it, where
+    /// [`Self::px_of`] wants only the length between them.
     fn px_region(&self, slot: MoveIdx, region: UiRegion) -> PixelRegion {
-        self.moves.resolve(slot, region).to_px(self.output_size)
+        self.moves.compose(slot, region).to_px(self.output_size)
     }
 
     /// A clean widget's retained answer, if that answer holds for a box of