fix: apply permutation in tangle composition type checking

Tangle[A,B] . Word[m] computed apply_perm_default m b into c
but then ignored it, returning TTangle(a,b) instead of TTangle(a,c).
The output boundary now correctly reflects the permutation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

compiler/lib/typecheck.ml

@@ -413,8 +413,7 @@ and infer_binop (op : binop) (t1 : ty) (t2 : ty) : ty =
                     word is wider than tangle output boundary"
           (pp_boundary a) (pp_boundary b) m;
       let c = apply_perm_default m b in
-      ignore c;
-      TTangle (a, b)
+      TTangle (a, c)
     | _ ->
       type_error "Cannot compose %s with %s" (pp_ty t1) (pp_ty t2)
     end