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>

IRIS.md

@@ -8,6 +8,37 @@ capability that moved. Small and trivial changes do not go here.
 An entry gives the date, what changed, why, and a short before/after where
 it helps judge the change without the session that made it. Newest first.
 
+## 2026-09-04: `Widget::draw` reports the size it used; `desired_width`/`desired_height` are gone
+
+A widget used to implement three methods (`draw`, `desired_width`,
+`desired_height`); it now implements one, `fn draw(&mut self, painter: &mut
+Painter) -> Size`, which draws into `painter.region()` and returns how much
+of it was used. Why: the two extra methods routinely re-simulated what
+`draw` was about to do anyway (`Span::desired_ortho` copied its own draw
+loop to get cross-axis sizing right) — one visit per widget per frame
+instead of up to three. A container that needs a child's size before
+placing it (alignment, centering) draws the child once at a provisional
+region, reads the returned `Size`, and calls the new `Painter::reposition`
+to move it into its final spot — an O(1) offset write, not a second draw. A
+widget whose drawn output never depends on the size it's given (a
+fixed-size `Rect`, a decoded `Image`) overrides the new `fn
+is_size_independent(&self) -> bool { false }` to `true`, which skips
+redrawing it when only its offered region changes shape.
+
+```rust
+// before
+fn draw(&mut self, painter: &mut Painter) { /* ... */ }
+fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len { /* ... */ }
+fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len { /* ... */ }
+
+// after
+fn draw(&mut self, painter: &mut Painter) -> Size { /* ... */ }
+```
+
+`SizeCtx` and `Cache` are gone with it — see `LAYOUT.md` for the full
+design, the move-offset mechanism this shipped alongside, and the file
+list.
+
 ## 2026-09-04: texture pipeline rebuilt off the binding array
 
 `Textures`/`TextureHandle`, `GlyphPrimitive`, and `UiRenderNode::new` all