Clean up namespaces in the analysis framework

- Wrap the forward-analysis framework in a Spa.Forward namespace so its
  generic names (analyze, result, joinAll, variablesAt, ...) no longer
  sit flat in Spa, matching the ConstAnalysis/SignAnalysis convention.
- Merge the split Graph namespace in Graphs.lean by relocating buildCfg.
- Use nested namespace Spa / Fixedpoint instead of Spa.Fixedpoint.

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

lean/Spa/Analysis/Sign.lean

@@ -5,6 +5,8 @@ import Spa.Showable
 
 namespace Spa
 
+open Forward
+
 inductive Sign where
   | plus
   | minus
@@ -215,7 +217,7 @@ instance eval_valid : ValidExprEvaluator SignLattice prog := by
 
 theorem analyze_correct {ρ : Env} (hrun : EvalStmt [] prog.rootStmt ρ) :
     ⟦ variablesAt prog.finalState (result SignLattice prog) ⟧ ρ :=
-  Spa.analyze_correct SignLattice prog hrun
+  Forward.analyze_correct SignLattice prog hrun
 
 end SignAnalysis