fix(verification): import Data.Nat for LTE in Idris2 trust proofs

AxiomPolicyOrdering and TrustLevelSoundness use LTE/LTESucc/LTEZero (from
Data.Nat) without importing it. Verified: idris2 --check exits 0 for both.

https://claude.ai/code/session_01UAqDQaMwpUqWHUSZekGZWv

Author: claude

verification/proofs/idris2/AxiomPolicyOrdering.idr

@@ -21,6 +21,8 @@
 
 module AxiomPolicyOrdering
 
+import Data.Nat
+
 %default total
 
 -- ==========================================================================

verification/proofs/idris2/TrustLevelSoundness.idr

@@ -20,6 +20,8 @@
 
 module TrustLevelSoundness
 
+import Data.Nat
+
 %default total
 
 -- ==========================================================================