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/Lattices.lean

@@ -4,6 +4,8 @@ import Spa.Interp
 
 namespace Spa
 
+namespace Forward
+
 variable (L : Type) [Lattice L] (prog : Program)
 
 abbrev VariableValues : Type := FiniteMap String L prog.vars
@@ -96,4 +98,6 @@ theorem interp_foldr {vs : VariableValues L prog}
     · exact interp_sup (Or.inl hvs)
     · exact interp_sup (Or.inr (ih hmem'))
 
+end Forward
+
 end Spa