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/Lattice/Prod.lean

@@ -6,7 +6,7 @@ section Unzip
 
 variable {α β : Type*} [PartialOrder α] [PartialOrder β]
 
-theorem LTSeries.exists_unzip (c : LTSeries (α × β)) :
+lemma LTSeries.exists_unzip (c : LTSeries (α × β)) :
     ∃ (c₁ : LTSeries α) (c₂ : LTSeries β),
       c₁.head = c.head.1 ∧ c₁.last = c.last.1 ∧
       c₂.head = c.head.2 ∧ c₂.last = c.last.2 ∧