fix: address 16 architecture design flaws across all crates

Phase 1 — Foundation:
- Constant-time token comparison via subtle::ConstantTimeEq (Fix 11)
- Structured error codes E001–E020 in new error_codes.rs (Fix 15)
- Remove dead envelope.capnp code and related types (Fix 16)

Phase 2 — Auth Hardening:
- Registration collision check via has_user_record() (Fix 5)
- Auth required on uploadHybridKey/fetchHybridKey RPCs (Fix 1)
- Identity-token binding at registration and login (Fix 2)
- Session token expiry with 24h TTL and background reaper (Fix 3)
- Bounded pending logins with 5-minute timeout (Fix 4)

Phase 3 — Resource Limits:
- Rate limiting: 100 enqueues/60s per token (Fix 6)
- Queue depth cap at 1000 + 7-day message TTL/GC (Fix 7)
- Partial queue drain via limit param on fetch/fetchWait (Fix 8)

Phase 4 — Crypto Fixes:
- OPAQUE KSF switched from Identity to Argon2id (Fix 10)
- Random AEAD nonce in hybrid KEM instead of HKDF-derived (Fix 12)
- Zeroize secret fields in HybridKeypairBytes (Fix 13)
- Encrypted client state files via QPCE format (Fix 9)

Phase 5 — Protocol:
- Commit fan-out to all existing members on invite (Fix 14)
- Add member_identities() to GroupMember

Breaking: existing OPAQUE registrations invalidated (Argon2 KSF).
Schema: added auth to hybrid key ops, identityKey to OPAQUE finish
RPCs, limit to fetch/fetchWait.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

schemas/node.capnp

@@ -24,19 +24,21 @@ interface NodeService {
   enqueue @2 (recipientKey :Data, payload :Data, channelId :Data, version :UInt16, auth :Auth) -> ();
 
   # Fetch and drain all queued payloads for the recipient.
-  fetch @3 (recipientKey :Data, channelId :Data, version :UInt16, auth :Auth) -> (payloads :List(Data));
+  # limit: max number of messages to return (0 = fetch all).
+  fetch @3 (recipientKey :Data, channelId :Data, version :UInt16, auth :Auth, limit :UInt32) -> (payloads :List(Data));
 
   # Long-poll: wait up to timeoutMs for new payloads, then drain queue.
-  fetchWait @4 (recipientKey :Data, channelId :Data, version :UInt16, timeoutMs :UInt64, auth :Auth) -> (payloads :List(Data));
+  # limit: max number of messages to return (0 = fetch all).
+  fetchWait @4 (recipientKey :Data, channelId :Data, version :UInt16, timeoutMs :UInt64, auth :Auth, limit :UInt32) -> (payloads :List(Data));
 
   # Health probe for readiness/liveness.
   health @5 () -> (status :Text);
 
   # Upload the hybrid (X25519 + ML-KEM-768) public key for sealed envelope encryption.
-  uploadHybridKey @6 (identityKey :Data, hybridPublicKey :Data) -> ();
+  uploadHybridKey @6 (identityKey :Data, hybridPublicKey :Data, auth :Auth) -> ();
 
   # Fetch a peer's hybrid public key (for post-quantum envelope encryption).
-  fetchHybridKey  @7 (identityKey :Data) -> (hybridPublicKey :Data);
+  fetchHybridKey  @7 (identityKey :Data, auth :Auth) -> (hybridPublicKey :Data);
 
   # ── OPAQUE password-authenticated registration ──────────────────────────
 
@@ -44,7 +46,7 @@ interface NodeService {
   opaqueRegisterStart  @8 (username :Text, request :Data) -> (response :Data);
 
   # Finish OPAQUE registration: client uploads sealed credential envelope.
-  opaqueRegisterFinish @9 (username :Text, upload :Data)  -> (success :Bool);
+  opaqueRegisterFinish @9 (username :Text, upload :Data, identityKey :Data)  -> (success :Bool);
 
   # ── OPAQUE password-authenticated login ─────────────────────────────────
 
@@ -52,7 +54,16 @@ interface NodeService {
   opaqueLoginStart  @10 (username :Text, request :Data)     -> (response :Data);
 
   # Finish OPAQUE login: client sends credential finalization, receives session token.
-  opaqueLoginFinish @11 (username :Text, finalization :Data) -> (sessionToken :Data);
+  opaqueLoginFinish @11 (username :Text, finalization :Data, identityKey :Data) -> (sessionToken :Data);
+
+  # ── P2P endpoint discovery ────────────────────────────────────────────────
+
+  # Publish this node's iroh endpoint address for P2P connectivity.
+  # nodeAddr is the serialized iroh NodeAddr (JSON or custom encoding).
+  publishEndpoint @12 (identityKey :Data, nodeAddr :Data, auth :Auth) -> ();
+
+  # Resolve a peer's iroh endpoint for direct P2P connection.
+  resolveEndpoint @13 (identityKey :Data, auth :Auth) -> (nodeAddr :Data);
 }
 
 struct Auth {