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/Analysis/Utils.lean

@@ -0,0 +1,15 @@
+/-
+Port of `Analysis/Utils.agda`. The `≼ᴼ-trans` module parameter lifts into the
+`Preorder` instance.
+-/
+import Spa.Lattice
+
+namespace Spa
+
+/-- Agda: `eval-combine₂`. -/
+theorem eval_combine₂ {O : Type*} [Preorder O] {combine : O → O → O}
+    (hmono : Monotone₂ combine) {o₁ o₂ o₃ o₄ : O}
+    (h₁ : o₁ ≤ o₃) (h₂ : o₂ ≤ o₄) : combine o₁ o₂ ≤ combine o₃ o₄ :=
+  le_trans (hmono.1 o₂ h₁) (hmono.2 o₃ h₂)
+
+end Spa