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/Language.lean

@@ -33,23 +33,23 @@ theorem trace {ρ : Env} (h : EvalStmt [] p.rootStmt ρ) :
 
 def vars : List String := p.rootStmt.vars.sort (· ≤ ·)
 
-theorem vars_nodup : p.vars.Nodup := Finset.sort_nodup _ _
+lemma vars_nodup : p.vars.Nodup := Finset.sort_nodup _ _
 
 def states : List p.State := p.cfg.indices
 
-theorem states_complete (s : p.State) : s ∈ p.states := p.cfg.mem_indices s
+lemma states_complete (s : p.State) : s ∈ p.states := p.cfg.mem_indices s
 
-theorem states_nodup : p.states.Nodup := p.cfg.nodup_indices
+lemma states_nodup : p.states.Nodup := p.cfg.nodup_indices
 
 def code (st : p.State) : List BasicStmt := p.cfg.nodes st
 
 def incoming (s : p.State) : List p.State := p.cfg.predecessors s
 
-theorem incoming_initialState_eq_nil : p.incoming p.initialState = [] :=
+lemma incoming_initialState_eq_nil : p.incoming p.initialState = [] :=
   Graph.wrap_predecessors_eq_nil p.rootStmt.cfg p.initialState
     (by rw [Graph.wrap_inputs]; exact List.mem_singleton_self _)
 
-theorem mem_incoming_of_edge {s₁ s₂ : p.State}
+lemma mem_incoming_of_edge {s₁ s₂ : p.State}
     (h : (s₁, s₂) ∈ p.cfg.edges) : s₁ ∈ p.incoming s₂ :=
   p.cfg.mem_predecessors_of_edge h