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/Bool.lean

@@ -17,11 +17,11 @@ def rank : Bool → ℕ
   | false => 0
   | true => 1
 
-theorem rank_strictMono : StrictMono rank := by
+lemma rank_strictMono : StrictMono rank := by
   intro a b hab
   cases a <;> cases b <;> revert hab <;> decide
 
-theorem boundedChains : BoundedChains Bool 1 := fun c => by
+lemma boundedChains : BoundedChains Bool 1 := fun c => by
   have h := LTSeries.head_add_length_le_nat (c.map rank rank_strictMono)
   rw [LTSeries.head_map, LTSeries.last_map, LTSeries.map_length] at h
   have h2 : rank c.last ≤ 1 := by cases c.last <;> simp [rank]