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/Forward/Adapters.lean

@@ -10,7 +10,7 @@ def updateVariablesFromExpression (k : String) (e : Expr)
     (vs : VariableValues L prog) : VariableValues L prog :=
   FiniteMap.generalizedUpdate id (fun _ vs => E.eval e vs) [k] vs
 
-theorem updateVariablesFromExpression_mono (k : String) (e : Expr) :
+lemma updateVariablesFromExpression_mono (k : String) (e : Expr) :
     Monotone (updateVariablesFromExpression (L := L) (prog := prog) k e) :=
   FiniteMap.generalizedUpdate_monotone monotone_id (fun _ => E.eval_mono e)
 
@@ -20,7 +20,7 @@ def evalBasicStmt (_ : prog.State) (bs : BasicStmt)
   | .assign k e => updateVariablesFromExpression k e vs
   | .noop => vs
 
-theorem evalBasicStmt_mono (s : prog.State) (bs : BasicStmt) :
+lemma evalBasicStmt_mono (s : prog.State) (bs : BasicStmt) :
     Monotone (evalBasicStmt (L := L) (prog := prog) s bs) := by
   cases bs with
   | assign k e => exact updateVariablesFromExpression_mono k e