Switch FiniteMap Fin n -> L representation

This helps automatically derive lattice laws for it

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lean/Spa/Showable.lean

@@ -30,7 +30,8 @@ instance {α : Type*} [Showable α] : Showable (AboveBelow α) :=
 instance {α β : Type*} {ks : List α} [Showable α] [Showable β] :
     Showable (FiniteMap α β ks) :=
   ⟨fun fm =>
-    "{" ++ fm.val.foldr (fun p rest => show' p.1 ++ " ↦ " ++ show' p.2 ++ ", " ++ rest) ""
+    "{" ++ (FiniteMap.toList fm).foldr
+        (fun p rest => show' p.1 ++ " ↦ " ++ show' p.2 ++ ", " ++ rest) ""
         ++ "}"⟩
 
 end Spa