Let OnResize answer for the window too, and delete the second rule

A resize had its own mechanism: `reads_output` recorded that a widget had
looked at the output, `update` scanned every active widget for one whose
`output_px` had moved, marked it and its whole reader chain, and
`resize_marks` kept those marks from counting as content dirtiness --
while `on_resize` answered the same question for every other box. Two
answers to "does this drawing survive its box changing length", and the
one that applied to the window ignored what the widget had declared.

With the output held as the root of the chain there is one question. A
resize offers the root widget its box again, `try_reuse` answers per axis
from `on_resize`, and `redraws_under` prices the subtree. Gone with it:
`reads_output`, `resized`, `resize_marks`, the scan, the eager reader
marking, and the shallowest-first branch in `redraw_updates`, which only
existed because resize marking worked differently -- the settle loop now
has one order.

Two things this needed. An unslotted widget may be reused when only its
parent's box changed length: it has nothing of its own to write, and what
it drew is a fraction of that box, so the slot above it already carries
the change. And `root_readers` holds the widgets whose size came from the
output rather than their own box -- `MaxSize` -- since no box of theirs
need have changed; they are marked per axis, from a set kept as they draw
rather than by scanning.

`a_resize_does_not_redraw_what_the_shader_can_move` now says `Scale`,
which is what it was always describing, and `a_resize_redraws_what_does
_not_scale` is its other half. `ReadsWidth` declares `Scale` across the
axis it does not read, so per-axis precision comes from the widget rather
than from which output axis it happened to touch.

Resize phase, seed 1 depth 8: 6.45M instructions per frame to 5.84M.

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

core/src/ui/active.rs

@@ -31,8 +31,6 @@ pub struct ActiveData {
     pub size_output_inputs: [bool; 2],
     /// The output dimensions against which those dependencies were observed.
     pub output_px: Vec2,
-    /// Output axes it read directly or while resolving its offered box.
-    pub reads_output: [bool; 2],
     /// The slot its primitives are positioned through: its own if its parent
     /// placed it, otherwise the nearest ancestor that has one.
     pub move_idx: MoveIdx,