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.

packages/bgm/src/schemas.ts

@@ -65,6 +65,7 @@ const setupValue = z.union([z.string(), z.array(z.string())]);
 const setupPlacement = z.object({
   path: z.string(),
   parts: setupValue,
+  facing: z.enum(['face', 'back', 'standing']).optional(),
 });
 
 const setupSchema = z.object({

packages/bgm/src/types.ts

@@ -157,6 +157,12 @@ export interface Surface {
 
 export type SetupValue = string | string[];
 
+/**
+ * How a part is oriented on the board. `face` lays it flat front-up, `back`
+ * flips it over front-down, and `standing` stands it on its bottom edge.
+ */
+export type Facing = 'face' | 'back' | 'standing';
+
 /**
  * One setup placement: move `parts` to `path`. Entries are applied in order,
  * so a part listed in a later entry ends up on that entry's path.
@@ -166,6 +172,8 @@ export interface SetupPlacement {
   path: string;
   /** Parts to place: a part id, a bare type (expands to all of that type), or a list of either. */
   parts: SetupValue;
+  /** Initial facing for the placed parts; defaults to `face`. */
+  facing?: Facing;
 }
 
 /** Seeds the state store: the enabled surfaces and an ordered list of placements. */