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-format.md

@@ -272,6 +272,10 @@ setup:
     parts: harbor:boat#fleet
   - path: /deck
     parts: harbor:card
+    facing: back
+  - path: /table
+    parts: harbor:token#wood
+    facing: standing
 ```
 
 `surfaces` lists the surfaces enabled at the start. A surface not listed is
@@ -287,6 +291,16 @@ the deck, then move these cards to the flop".
 either. A bare type expands to all parts of that type during game state
 initialization.
 
+`facing` sets how the placed parts are oriented on the board, defaulting to
+`face`:
+
+- `face` — lay flat, front up, resting on the bottom face.
+- `back` — lay flat, front down (flipped over), resting on the top face.
+- `standing` — stand upright on the bottom edge, front texture still showing.
+
+A part's `facing` is seeded into the game state and can change at runtime; it
+only affects orientation, never the part's texture.
+
 ---
 
 ## 4. Concepts