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/Adapters.lean

@@ -2,6 +2,8 @@ import Spa.Analysis.Forward.Evaluation
 
 namespace Spa
 
+namespace Forward
+
 variable {L : Type} [Lattice L] {prog : Program} [E : ExprEvaluator L prog]
 
 def updateVariablesFromExpression (k : String) (e : Expr)
@@ -51,4 +53,6 @@ instance ExprEvaluator.toStmtEvaluator_valid [LatticeInterpretation L]
           (fun hmem => hne (List.mem_singleton.mp hmem)) hk'l₀
       exact hvs _ _ hk'l' _ hmem'
 
+end Forward
+
 end Spa