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/tabletop/src/index.ts

@@ -8,6 +8,7 @@ export {
   fallbackShape,
   traceToShape,
   traceToUvBounds,
+  facingTransform,
   MM_TO_WORLD,
 } from './part.js';
 export { resolveAssetUrl, assetUrl, traceImage } from './http.js';