Improve performance by caching CFG building

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

lean/Spa/Language.lean

@@ -10,11 +10,19 @@ namespace Spa
 
 structure Program where
   rootStmt : Stmt
+  /-- A memoized copy of the control-flow graph. This field is an
+      implementation detail to avoid re-computing `Graph.wrap` and `Stmt.cfg`
+      every time the program's control flow graph is needed -/
+  cfgCache : Thunk Graph := Thunk.mk fun _ => Graph.wrap rootStmt.cfg
 
 namespace Program
 
 variable (p : Program)
 
+-- Runtime implementation of `cfg`: read the memoized graph.
+private def cfgImpl : Graph := p.cfgCache.get
+
+@[implemented_by cfgImpl]
 def cfg : Graph := Graph.wrap p.rootStmt.cfg
 
 abbrev State : Type := p.cfg.Index