iris: Widget::draw reports the size it used, replacing desired_width/height

Implements LAYOUT.md end to end: one fn draw(&mut self, &mut Painter) ->
Size replaces draw + desired_width/desired_height on every widget in
iris/src/widget/, SizeCtx and Cache are deleted, and a moved widget
(Scroll, Offset) costs one move_offsets write resolved by a shared
resolve_move WGSL function in both shader stages -- O(1) regardless of
how many primitives are in its subtree, measured at 500 in the new
iris/src/layout_tests.rs (a plain unit test: UiRenderState touches no
GPU or window).

Five real bugs surfaced only by diffing iris/run-headless.sh screenshots
against the pre-change tree and are written up in LAYOUT.md's
"Deviations found during implementation": Aligned's provisional draw
composing painter.region() a second time through widget_within; Sized/
MaxSize reporting a capped size while still painting their child
unconstrained (fine under the old two-pass model, wrong once a parent
like Aligned draws before knowing the final size); a widget's
move_offsets parent link being unreadable from self.active while its
own ActiveData is still mid-construction; Painter::reposition needing
the child's *painted* footprint (its reported size, top-left anchored)
rather than its offered region; and a widget's move slot needing to be
reused in place across redraws, with its delta reset, rather than
reallocated.

All four iris/examples render pixel-identical to the pre-change tree.
cargo fmt/clippy/test clean across the workspace (18 tests: 14
pre-existing plus 4 new).

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

RUST.md

@@ -39,6 +39,24 @@ session spending an afternoon on them again.
 - **Done**: E0 (toolchain), E1 (Masonry on android-view, which found the
   keyboard gap — now explained, see below), I0a, I0b (iris builds on a
   pinned nightly and runs), I1 (parley + glyph atlas).
+- **Done, 2026-09-04: the `Widget::draw`/layout redesign (LAYOUT.md).**
+  `desired_width`/`desired_height`/`SizeCtx`/`Cache` are gone; every widget
+  in `iris/src/widget/` implements one `fn draw(&mut self, &mut Painter) ->
+  Size`. A moved widget (`Scroll`, `Offset`) now costs one
+  `move_offsets` write resolved by a shared `resolve_move` WGSL function in
+  both shader stages, independent of how many primitives are in its
+  subtree — measured at 500 in `iris/src/layout_tests.rs`, which also
+  covers the unchanged-frame, hit-test-after-move and mask-follows-move
+  pass conditions as plain unit tests (no GPU or window needed, since
+  `UiRenderState` touches neither). All four examples render
+  pixel-identically to before the change. See LAYOUT.md's "Deviations
+  found during implementation" for five real bugs the design's first draft
+  did not anticipate — worth reading before touching `Aligned`, `Sized`,
+  `MaxSize`, `Scroll`, or the move-slot lifecycle again. Not done: a
+  pixel-level screenshot check of a `Masked`-wrapped `Scroll` (no example
+  builds one yet — the numeric check in `layout_tests.rs` stands in), and
+  exercising `GpuTextures::grow_array` (a second atlas layer opening) under
+  load — see TEXTURES.md.
 - **E1's keyboard gap is Masonry's `as_input_connection` returning `None`
   (a TODO), not android-view or `EditorInfo`.** android-view's own demo
   implements the `InputConnection` trait over a parley editor and gets