Lean migration: Phase 7 (Sign + Constant analyses, executable)

- Spa.Showable: port of Showable.agda (quoted strings, map format) for
  output parity
- Spa.Analysis.Utils: eval_combine₂
- Spa.Lattice.AboveBelow.le_cases: order of the flat lattice by cases
- Spa.Analysis.Sign / Spa.Analysis.Constant: the four monotonicity
  POSTULATES from the Agda files are now proved theorems (via le_cases);
  interpretations, evaluator validity, analyze_correct per analysis
- Main + lake exe spa: runs both analyses on the Agda test program;
  constant analysis folds unknown=0, sign analysis gives unknown=⊤

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

lean/Spa.lean

@@ -16,3 +16,7 @@ import Spa.Analysis.Forward.Lattices
 import Spa.Analysis.Forward.Evaluation
 import Spa.Analysis.Forward.Adapters
 import Spa.Analysis.Forward
+import Spa.Showable
+import Spa.Analysis.Utils
+import Spa.Analysis.Sign
+import Spa.Analysis.Constant