Specify ZIP 318 note-preparation expiry

Require note-preparation transactions to derive their expiry height from their scheduled broadcast height using the same canonical rule as migration transactions.

Co-Authored-By: OpenAI Codex (GPT-5) <noreply@openai.com>

Author: ValarDragon

zips/zip-0318.md

@@ -419,6 +419,11 @@ note distribution, and the distinction is not visible on-chain (see
 * Note preparation MUST be implemented as one or more wallet-internal
   send-to-self transactions with no external receiver.
 
+* Each note-preparation transaction SHOULD use the canonical expiry height
+  specified for migration transactions in
+  [Canonical migration transaction structure](#canonicalmigrationtransactionstructure),
+  computed from the note-preparation transaction's scheduled broadcast height.
+
 * For mobile wallets, proving time is a concern. For that reason, and also for
   indistinguishability, Note preparation transactions SHOULD be constructed 
   such that each preparation transaction contains exactly