Make a declared length one that cannot carry a share

`declared_lens` filtered `leftover` out of both its sources and every
consumer then re-dropped it, so the rule lived in two filters and a comment.
A declaration is a `Len`: `LayoutLen::declared` states the rule once and both
sources go through it, and `Declared` replaces the bare two-element array on
`ActiveData` and in four signatures.

The two sources stay one value deliberately. A rule decides the child's box;
a hint only promises what it will report -- but `size_hint` is by contract an
exact answer with no painter context, and `hints_agree` fails a widget that
draws something else, so narrowing the box to a hint cannot change what is
drawn. Every consumer asks about the length, never which said it.

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

core/src/ui/render_state.rs

@@ -1,9 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::{
-    ActiveData, Axis, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx, Moves,
-    Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan,
-    UiVec2, Weight, WidgetId, Widgets,
+    ActiveData, Axis, Declared, DrawLayers, IdLike, LayoutHolds, LayoutLen, Len, MaskIdx, MoveIdx,
+    Moves, Painter, PixelRegion, PlaceDesc, PxVec2, Rel, Size, StrongWidget, UiRegion, UiRsc,
+    UiSpan, UiVec2, Weight, WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
@@ -868,7 +868,7 @@ impl UiRenderState {
                 children: Vec::new(),
                 size_deps: Vec::new(),
                 move_idx: info.parent_move,
-                declared: [None; 2],
+                declared: Declared::NONE,
                 own_align: rsc.widgets().alignment(id),
                 parent_move: info.parent_move,
                 mask: info.mask,