ludic.ui: the pointer looks through a box with no size; a control's parts answer :hover

From the user's play, reproduced on Maroon Lake's customisation screen with the pointer scripted over
each control (the lab's R3D_CLICKS): nothing on it was ever pressed or hovered.

- A screen's root <div> round a positioned panel (a component's root holding the kit Screen's
  modal) lays out to 0x0, and the hit test (fr_pick) and :hover (fr_under) only walked into a child
  whose box held the point - so nothing under it was reachable. A box with no size holds no point and
  clips nothing: both look through it now (fr_empty), without hovering it.
- A control's parts (a select's < and >, a range's thumb) are made by ct_part, which never set
  `hovered`: `select .ui-prev:hover` could not match anywhere. It is set as an element's is.

Golden ui_pointer_through: the panel's button pressed and hovered through the empty root, and a
select's > hovered (before this: pressed 0, hovered 0). The 36 ui examples pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.ui/controls_build.ludic

@@ -7,6 +7,7 @@
 function ct_part(ui_st: mut UiState, n: UiNode, cls: string, text: string) -> UiNode {
   let p = nd_take(ui_st)
   p.key = mm_join(ui_st, n.key, "/", cls)
+  p.hovered = fr_was_hovered(ui_st, p.key)     # a part answers :hover as an element does (a select's < and >)
   p.tag = "span"
   p.env = n.env
   p.sheet = n.sheet