Add scoped quotation syntax for object-language programs

Introduce [spa_e| ... ] for Expr and [spa| ... ] for Stmt, scoped to the
Spa namespace via a dedicated syntax category and macro_rules. This removes
the deeply nested .andThen / .basic (.assign ...) boilerplate when writing
programs; Main.lean's test programs are rewritten to use it.

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

lean/Spa.lean

@@ -7,6 +7,7 @@ import Spa.Lattice.AboveBelow
 import Spa.Lattice.IterProd
 import Spa.Lattice.FiniteMap
 import Spa.Language.Base
+import Spa.Language.Notation
 import Spa.Language.Semantics
 import Spa.Language.Graphs
 import Spa.Language.Traces