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/Forward/Evaluation.lean

@@ -2,6 +2,8 @@ import Spa.Analysis.Forward.Lattices
 
 namespace Spa
 
+namespace Forward
+
 variable (L : Type) [Lattice L] (prog : Program)
 
 class StmtEvaluator where
@@ -23,4 +25,6 @@ class ValidStmtEvaluator [E : StmtEvaluator L prog] [LatticeInterpretation L] :
     {bs : BasicStmt},
     EvalBasicStmt ρ₁ bs ρ₂ → ⟦ vs ⟧ ρ₁ → ⟦ E.eval s bs vs ⟧ ρ₂
 
+end Forward
+
 end Spa