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.lean

@@ -5,6 +5,8 @@ import Spa.Fixedpoint
 
 namespace Spa
 
+namespace Forward
+
 variable {L : Type} [Lattice L] {prog : Program} [E : StmtEvaluator L prog]
 
 def updateVariablesForState (s : prog.State) (sv : StateVariables L prog) :
@@ -116,4 +118,6 @@ theorem analyze_correct {ρ : Env} (hrun : EvalStmt [] prog.rootStmt ρ) :
     ⟦ variablesAt prog.finalState (result L prog) ⟧ ρ :=
   walkTrace interp_joinForKey_initialState (prog.trace hrun)
 
+end Forward
+
 end Spa