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.

native/include/lua/layout.h

@@ -32,7 +32,6 @@ enum Type : uint8_t { BOX, TEXT, BUTTON, CUSTOM };
 
 enum Flag : uint8_t {
   ROW = 1 << 0,         // main axis is horizontal
-  CAPTURE = 1 << 1,     // swallows the taps its children missed
   INTERACTIVE = 1 << 2, // has an on_press
   DIRTY = 1 << 3,
   PRESSED = 1 << 4,
@@ -204,8 +203,10 @@ public:
     return error == nullptr;
   }
 
-  // Deepest interactive node wins, so a tappable child beats its tappable
-  // parent. The list is singly linked, so "last match walking forward" stands
+  // Deepest node by geometry wins, so a child beats its parent and a later sibling
+  // beats an earlier one it overlaps. Whether that node responds to a press or a pan is
+  // dispatch's business, which bubbles up from here -- hit-testing is not fused with who
+  // handles the gesture. The list is singly linked, so "last match walking forward" stands
   // in for "first match walking backward"; they name the same node.
   uint16_t hit(uint16_t id, int px, int py) const {
     const Node& n = nodes[id];
@@ -222,9 +223,7 @@ public:
     }
     if (found != NONE)
       return found;
-    if (n.flags & CAPTURE)
-      return id;
-    return (n.flags & INTERACTIVE) ? id : NONE;
+    return id;
   }
 
   bool scrolls(uint16_t id) const {