add canonical sigchain bundle format

docs/adr/0018-sshsigchain.md

@@ -36,7 +36,12 @@ SSHSIGCHAIN v1 is redesigned before deployment:
 - cached ancestors are never replaced by older heads and incomparable verified
   histories fail as forks; and
 - a policy-change head is witnessed under the preceding policy before the new
-  policy governs descendants.
+  policy governs descendants;
+- canonical `.sscb` bundles deterministically store records, separately
+  discloseable payloads, claims, and receipts without embedding root trust;
+- JSONL is noncanonical human-facing interchange; and
+- distribution backends return untrusted bundle bytes through one interface,
+  with a static HTTP profile that requires no server application.
 
 The generic core defines backend interfaces and deterministic policy behavior,
 not Nostr, HTTP, blockchain, or transparency-log clients. Those adapters belong
@@ -60,5 +65,6 @@ draft. No migration parser or version alias is retained.
 Rollback protection still depends on persistent local accepted heads and the
 operator's anchor policy. A Nostr relay, mutable HTTP URL, blockchain, or
 transparency service has only the guarantees its receipt verifier and deployment
-actually establish. Concrete publication, durable accepted-head storage, and
+actually establish. Canonical bundle conversion and verification exist, while
+automatic HTTP/Iroh fetching, durable accepted-head storage, and
 cross-implementation vectors remain follow-up work.