Build menu: hover popup with full info, top-left card hidden while building

The short description row became a status row for the selected piece
(variant, Replace, ghosts, messages). Gamepad shows the selected tile's popup.

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

docs/DESIGN.md

@@ -146,7 +146,9 @@ above, including:
 - **Pick** (middle mouse) selects the piece under the cursor with its direction,
   wall material and filter settings. G is unbound, as the owner requested.
 - **The build menu** is a bar of tabs and icon tiles along the bottom: Structure, Moving,
-  Liquids, Machines, Molds. The owner wants more changes (see NOTES.md).
+  Liquids, Machines, Molds. Hover a tile for its full description, cost and placement help;
+  gamepad players see the selected tile’s popup. The top-left card is hidden while building,
+  with variants, ghosts and status kept in the bar so the world stays clear.
 - **No length limit when dragging.** Blocks out of the bot's reach become ghosts that it builds
   when it gets there.
 - **Pipes are thin:** sand, water and anything else loose passes them. Pumps and outlets are