Lean migration: Phase 6 (forward analysis framework)

- Spa.Analysis.Forward.Lattices: VariableValues/StateVariables (FiniteMap
  instantiations), fixed heights, variablesAt, joinForKey/joinAll, interpV
  and its sup/foldr lemmas
- Spa.Analysis.Forward.Evaluation: StmtEvaluator/ExprEvaluator + validity
  (the Agda Valid* instance records become plain Props)
- Spa.Analysis.Forward.Adapters: expr-to-stmt evaluator adapter + validity
- Spa.Analysis.Forward: updateAll, analyze, result (least fixpoint via the
  gas-based Fixedpoint), walkTrace, analyze_correct — the framework's main
  soundness theorem

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lean/Spa.lean

@@ -12,3 +12,7 @@ import Spa.Language.Graphs
 import Spa.Language.Traces
 import Spa.Language.Properties
 import Spa.Language
+import Spa.Analysis.Forward.Lattices
+import Spa.Analysis.Forward.Evaluation
+import Spa.Analysis.Forward.Adapters
+import Spa.Analysis.Forward