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/Language/Graphs.lean

@@ -89,17 +89,6 @@ def singleton (bss : List BasicStmt) : Graph where
 def wrap (g : Graph) : Graph :=
   singleton [] ⤳ g ⤳ singleton []
 
-end Graph
-
-open Graph in
-def buildCfg : Stmt → Graph
-  | .basic bs => Graph.singleton [bs]
-  | .andThen s₁ s₂ => buildCfg s₁ ⤳ buildCfg s₂
-  | .ifElse _ s₁ s₂ => buildCfg s₁ ∙ buildCfg s₂
-  | .whileLoop _ s => Graph.loop (buildCfg s)
-
-namespace Graph
-
 variable (g : Graph)
 
 def indices : List g.Index := List.finRange g.size
@@ -123,4 +112,11 @@ theorem edge_of_mem_predecessors {idx₁ idx₂ : g.Index}
 
 end Graph
 
+open Graph in
+def buildCfg : Stmt → Graph
+  | .basic bs => Graph.singleton [bs]
+  | .andThen s₁ s₂ => buildCfg s₁ ⤳ buildCfg s₂
+  | .ifElse _ s₁ s₂ => buildCfg s₁ ∙ buildCfg s₂
+  | .whileLoop _ s => Graph.loop (buildCfg s)
+
 end Spa