Fix off-by-one error in variant sort mapping

This didn’t actually have any negative effect because of how it was implemented. But it only happened to work right :D

src/lib/offsets.js

@@ -190,7 +190,7 @@ export class Offsets {
 
     return {
       ...this.create('variants'),
-      variants: 1n << this.reservedVariantBits,
+      variants: this.variantOffsets.get(variant),
     }
   }