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>

crates/quicnprotochat-core/src/group.rs

@@ -361,6 +361,21 @@ impl GroupMember {
         self.group.as_ref()
     }
 
+    /// Return the identity (credential) bytes of all current group members.
+    ///
+    /// Each entry is the raw credential payload (Ed25519 public key bytes)
+    /// extracted from the member's MLS leaf node.
+    pub fn member_identities(&self) -> Vec<Vec<u8>> {
+        let group = match self.group.as_ref() {
+            Some(g) => g,
+            None => return Vec::new(),
+        };
+        group
+            .members()
+            .map(|m| m.credential.identity().to_vec())
+            .collect()
+    }
+
     // ── Private helpers ───────────────────────────────────────────────────────
 
     fn make_credential_with_key(&self) -> Result<CredentialWithKey, CoreError> {