Bring wire protocol into full YAW/2 spec compliance

Five interop issues fixed against PROTOCOL.md:

- Join signature now covers nonce_raw || net_ascii (64-char UTF-8 hex
  string) as specified in §5.1, not net_raw_bytes. Both anchor server
  and client updated to match.

- Chat wire fields renamed to spec names: text/ts (Unix ms int64)
  replacing body/sent_at (ISO timestamp). Flat layout on PeerMessage
  matches §8 exactly; store and TUI updated accordingly.

- Direct messages now use the spec "pm" type (flat {type,mid,text,ts})
  instead of chat+to. Receiver reconstructs a ChatMessage with
  dm:<short-id> room for IPC/storage. §8 compliant.

- File transfer message types changed to spec hyphenated names:
  file-offer, file-accept, file-cancel, file-done with spec field
  names (name/size not filename/size_bytes). §9 compliant.

- DataChannel open-race (§14 gotcha #3) fixed with sync.Once: doOpen
  fires on OnOpen callback or immediately if the channel is already
  open when WireDataChannel is called (answerer race).

Also fixes two bugs found during testing:

- mid was missing from outgoing wire messages, causing all received
  messages to arrive with mid="" and collide on the UNIQUE DB
  constraint. mid is now included on all sent chat/pm messages; a
  random mid is generated for any received message that omits it.

- Test scripts hardened: kill -9 + active lsof polling replaces blind
  sleep for port cleanup; join_network sent before peer_field queries
  (local_peer is now network-scoped and nil until joined).

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

internal/store/store.go

@@ -57,10 +57,11 @@ func (s *Store) Close() error {
 // SaveMessage persists a chat message. Duplicate mids are silently ignored
 // (INSERT OR IGNORE), so calling this more than once is safe.
 func (s *Store) SaveMessage(msg *proto.ChatMessage) error {
+	sentAt := time.UnixMilli(msg.Ts).UTC()
 	_, err := s.db.Exec(
 		`INSERT OR IGNORE INTO messages (mid, room, from_peer, body, sent_at)
 		 VALUES (?, ?, ?, ?, ?)`,
-		msg.Mid, msg.Room, string(msg.From), msg.Body, msg.SentAt.UTC(),
+		msg.Mid, msg.Room, string(msg.From), msg.Text, sentAt,
 	)
 	return err
 }
@@ -96,12 +97,12 @@ func (s *Store) RecentMessages(room string, limit int) ([]proto.ChatMessage, err
 		var m proto.ChatMessage
 		var from string
 		var sentAt time.Time
-		if err := rows.Scan(&m.Mid, &from, &m.Body, &sentAt); err != nil {
+		if err := rows.Scan(&m.Mid, &from, &m.Text, &sentAt); err != nil {
 			return nil, err
 		}
 		m.From = proto.PeerID(from)
 		m.Room = room
-		m.SentAt = sentAt
+		m.Ts = sentAt.UnixMilli()
 		msgs = append(msgs, m)
 	}
 	// Reverse so oldest-first.