feat: add facing orientation to parts

Replace the face/back boolean with a three-state facing (face, back,
standing) seeded from setup placements. Parts now orient about the
anchor at the resting face/edge, so back-down parts sit on the table
instead of below it and standing parts rest on their bottom edge.

docs/bgm-tabletop-plan.md

@@ -86,7 +86,7 @@ interface GameState {
 interface PartState {
   path: string;  // the path key this part is on
   index: number; // the part's position in its path's stack
-  face: boolean; // whether the part's face is up
+  facing: 'face' | 'back' | 'standing'; // how the part is oriented on the board
 }
 ```