Add single-use registration cancellation

workplans/EMAIL-WP-0004-transactional-mail-delivery-service.md

@@ -8,7 +8,7 @@ status: active
 owner: codex
 topic_slug: netkingdom
 created: "2026-08-08"
-updated: "2026-08-08"
+updated: "2026-08-10"
 depends_on:
   - NK-WP-0024
 state_hub_workstream_id: "a37e5e4d-090a-4a75-88d2-aacd0c0fd235"
@@ -68,6 +68,13 @@ SQLite retains a SHA-256 digest, evidence is purpose/binding scoped, expiry is
 enforced, and consumption is atomic and single-use. All 23 repository tests
 pass.
 
+2026-08-10 cancellation increment: verification messages now include a fixed
+portal cancellation link using the same opaque handle. The SQLite store keeps
+only its digest and atomically records either consumption or cancellation, so
+verification and cancellation invalidate one another and replay fails closed.
+Purpose-bound cancellation evidence lets user-engine abandon the matching
+registration without accepting browser identity claims. All 24 tests pass.
+
 ## T03 - Establish custody and deploy the service
 
 ```task