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/active.rs

@@ -1,5 +1,5 @@
 use crate::{
-    LayerId, LayoutHolds, LayoutLen, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
+    Declared, LayerId, LayoutHolds, MaskIdx, MoveIdx, PlaceDesc, RegionAlign, RetainedPrimitive,
     Size, TextureHandle, UiRegion, UiVec2, WidgetId,
 };
 
@@ -59,7 +59,7 @@ pub struct ActiveData {
     /// The declared lengths whoever drew this widget resolved into its rel base.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
-    pub declared: [Option<LayoutLen>; 2],
+    pub declared: Declared,
     /// Its alignment when it was last drawn, which a change to the property
     /// is found against.
     pub own_align: RegionAlign,