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/Analysis/Reaching.lean

@@ -23,7 +23,7 @@ def eval (s : prog.State) :
     FiniteMap.generalizedUpdate id (fun _ _ => genSet prog s) [k] vs
   | .noop, vs => vs
 
-theorem eval_mono (s : prog.State) (bs : BasicStmt) :
+lemma eval_mono (s : prog.State) (bs : BasicStmt) :
     Monotone (eval prog s bs) := by
   cases bs with
   | assign k e =>