feat(player): add mobile directional pad controls

ARCHITECTURE.md

@@ -315,10 +315,13 @@ The browser application has four presentation modes:
 3. player match view;
 4. observer full-map view.
 
-Player controls support keyboard movement, touch swipes, and an explicit
-drop-key action. The UI renders only server-provided visible state. The observer
-view supports fitting, panning, or zooming the complete `32 x 32` map without
-changing server authority.
+Player controls support keyboard movement, avatar drag, touch swipes, an
+explicit four-button directional pad, and an explicit drop-key action. Every
+movement input uses the same bounded latest-intent path and sends only a
+cardinal intention; the browser does not predict a movement result. The UI
+renders only server-provided visible state. The observer view supports fitting,
+panning, or zooming the complete `32 x 32` map without changing server
+authority.
 
 `docs/UI_STYLE.md` is the canonical browser presentation contract. The player
 layout supports a minimum `412 x 916` CSS-pixel viewport. Observer mode requires