feat: add delivery sequence numbers + major server/client refactor

Delivery sequence numbers (MLS epoch ordering fix):
- schemas/node.capnp: add Envelope{seq,data} struct; enqueue returns seq:UInt64;
  fetch/fetchWait return List(Envelope) instead of List(Data)
- storage.rs: Store trait enqueue returns u64; fetch/fetch_limited return
  Vec<(u64, Vec<u8>)>; FileBackedStore gains QueueMapV3 with per-inbox seq
  counters and V2→V3 on-disk migration
- migrations/002_add_seq.sql: seq column, delivery_seq_counters table, index
- sql_store.rs: atomic UPSERT counter via RETURNING, ORDER BY seq, SCHEMA_VERSION→3
- node_service/delivery.rs: builds Envelope list; returns seq from enqueue
- client/rpc.rs: enqueue→u64, fetch_all/fetch_wait→Vec<(u64,Vec<u8>)>
- client/commands.rs: sort-by-seq before MLS processing; retry loop in cmd_recv
  and receive_pending_plaintexts for correct epoch ordering

Server refactor:
- Split monolithic main.rs into node_service/{mod,delivery,auth_ops,key_ops,p2p_ops}
- Add auth.rs (token validation, rate limiting), config.rs, metrics.rs, tls.rs
- Add SQL migrations runner (001_initial.sql, 002_add_seq.sql)
- OPAQUE PAKE login/registration, sealed-sender mode, queue depth limit (1000)

Client refactor:
- Split lib.rs into client/{commands,rpc,state,retry,hex,mod}
- Add cmd_whoami, cmd_health, cmd_check_key, cmd_ping subcommands
- Add cmd_register_user, cmd_login (OPAQUE), cmd_refresh_keypackage
- Hybrid PQ envelope (X25519 + ML-KEM-768) on all send/recv paths
- E2E test suite expanded

Other:
- quicnprotochat-gui: Tauri 2 desktop GUI skeleton (backend + HTML UI)
- quicnprotochat-p2p: iroh-based P2P transport stub
- quicnprotochat-core: app_message, hybrid_crypto modules; GroupMember API updates
- .github/workflows/size-lint.yml: binary size regression check
- docs: protocol comparison, roadmap updates, fully-operational checklist

schemas/node.capnp

@@ -20,15 +20,18 @@ interface NodeService {
   # channelId : Optional channel identifier (empty for default). A 16-byte UUID
   #             is recommended for 1:1 channels.
   # version   : Schema/wire version. Must be 1.
-  enqueue @2 (recipientKey :Data, payload :Data, channelId :Data, version :UInt16, auth :Auth) -> ();
+  # Returns the monotonically increasing per-inbox sequence number assigned to this message.
+  enqueue @2 (recipientKey :Data, payload :Data, channelId :Data, version :UInt16, auth :Auth) -> (seq :UInt64);
 
   # Fetch and drain all queued payloads for the recipient.
   # 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));
+  # Returns envelopes with per-inbox sequence numbers for ordered MLS processing.
+  fetch @3 (recipientKey :Data, channelId :Data, version :UInt16, auth :Auth, limit :UInt32) -> (payloads :List(Envelope));
 
   # Long-poll: wait up to timeoutMs for new payloads, then drain queue.
   # 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));
+  # Returns envelopes with per-inbox sequence numbers for ordered MLS processing.
+  fetchWait @4 (recipientKey :Data, channelId :Data, version :UInt16, timeoutMs :UInt64, auth :Auth, limit :UInt32) -> (payloads :List(Envelope));
 
   # Health probe for readiness/liveness.
   health @5 () -> (status :Text);
@@ -70,3 +73,10 @@ struct Auth {
   accessToken @1 :Data; # opaque bearer token issued at login
   deviceId @2 :Data;    # optional UUID bytes for auditing/rate limiting
 }
+
+# A delivery envelope pairing a per-inbox sequence number with an opaque payload.
+# Clients sort by `seq` before processing to guarantee MLS commit ordering.
+struct Envelope {
+  seq  @0 :UInt64; # monotonically increasing per-inbox counter (assigned by server)
+  data @1 :Data;   # opaque payload (hybrid-encrypted MLS message)
+}