Start working on notation for formalization

Per convention, create a new instance for 'interpretable' thing,
with an fundep'ed semantic domain. I feel at peace with this notation
even though it conflicts with Mathlib's quotients.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lean/Spa/Interp.lean

@@ -0,0 +1,10 @@
+import Mathlib.Tactic.TypeStar
+
+namespace Spa
+
+class Interp (α : Type*) (dom : outParam Type*) where
+  interp : α → dom
+
+notation:max (priority := high) "⟦" v "⟧" => Interp.interp v
+
+end Spa