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/Fixedpoint.lean

@@ -1,6 +1,8 @@
 import Spa.Lattice
 
-namespace Spa.Fixedpoint
+namespace Spa
+
+namespace Fixedpoint
 
 open FiniteHeightLattice (height)
 
@@ -49,4 +51,6 @@ theorem aFix_le (f : α → α) (hf : Monotone f)
     {a : α} (ha : a = f a) : aFix f hf ≤ a :=
   doStep_le f hf ha _ _ _ _ (by simpa using FiniteHeightLattice.bot_le α a)
 
-end Spa.Fixedpoint
+end Fixedpoint
+
+end Spa