Adopt lemma as the default keyword

Convert every theorem to lemma (mathlib's default) except the headline results a
reader of each module seeks out: analyze_correct (Forward/Sign/Constant),
aFix_eq/aFix_le (Fixedpoint), trace (Language), and Stmt.cfg_sufficient
(Language/Properties). lemma and theorem are interchangeable keywords, so no
references change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lean/Spa/Fixedpoint.lean

@@ -34,7 +34,7 @@ theorem aFix_eq (f : α → α) (hf : Monotone f) :
     aFix f hf = f (aFix f hf) :=
   (fix f hf).2
 
-theorem doStep_le (f : α → α) (hf : Monotone f)
+lemma doStep_le (f : α → α) (hf : Monotone f)
     {b : α} (hb : b = f b) :
     ∀ (g : ℕ) (c : LTSeries α) (hlen : c.length + g = height (α := α) + 1)
       (hle : c.last ≤ f c.last), c.last ≤ b →