feat(ui): tree-owned scroll gestures and geometry hit-testing

Drag, flick and tap-vs-scroll now live on the scrollX/scrollY flag in
ui.lua, so any scroll box pans with no app code. tree.hit returns the
deepest node by geometry and dispatch bubbles to the nearest handler,
dropping the now-unused CAPTURE flag. keyboard moves in as ui.keyboard,
and embed compiles nested lib dirs to dotted module names.

lua/api/features/screen/tree.lua

@@ -16,7 +16,6 @@
 ---@field justify? "start"|"center"|"end"|"between"
 ---@field row? boolean
 ---@field at? table Absolute-position fields.
----@field capture? boolean
 ---@field interactive? boolean
 ---@field label? string
 ---@field font? ScreenFont