Stop keeping what nothing reads back

`ActiveData::size_deps` was written on every draw, cleared on every undraw,
and read nowhere: a `Vec<WidgetId>` per active widget for a list only the
`Painter`'s own copy is used from, in `draw_at`, before the record is built.
What it looked like it was for -- reaching a widget whose size was read --
is already done there, by recording whoever asked about a child it did not
draw.

`SizeRule::apply` had no caller and would have been wrong if it found one:
it answers with the rule's own length, where `draw_at` resolves a fraction
against the rel base first. One rule, applied in one place.

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

core/src/ui/active.rs

@@ -51,8 +51,6 @@ pub struct ActiveData {
     /// An owned mask holds one reference independently of its primitives.
     pub mask_region: Option<UiRegion>,
     pub children: Vec<WidgetId>,
-    /// The children whose size this widget read while drawing.
-    pub size_deps: Vec<WidgetId>,
     /// The movable region its primitives are positioned through: its own when
     /// opted in, otherwise the nearest ancestor's.
     pub move_idx: MoveIdx,

core/src/ui/render_state.rs

@@ -489,7 +489,6 @@ impl UiRenderState {
             primitives,
             mask_region,
             children,
-            size_deps,
             declared: rsc.widgets().declared_lens(id),
             own_align: rsc.widgets().alignment(id),
             move_idx,
@@ -800,7 +799,6 @@ impl UiRenderState {
         }
         // After the descendants, whose slots name this one as their parent.
         self.drop_slot(id);
-        active.size_deps.clear();
         active.drawn = false;
         self.active.insert(id, active);
     }
@@ -844,7 +842,6 @@ impl UiRenderState {
                 primitives: Vec::new(),
                 mask_region: None,
                 children: Vec::new(),
-                size_deps: Vec::new(),
                 move_idx: info.parent_move,
                 declared: Declared::NONE,
                 own_align: rsc.widgets().alignment(id),