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/test/ui.lua

@@ -92,6 +92,9 @@ tree = {
   attach = function(parent, child)
     nodes[child].parent = parent
   end,
+  getParent = function(id)
+    return nodes[id].parent
+  end,
   setSize = function() end,
   setStyle = function(id, style)
     for key, value in pairs(style) do