feat: EXT-007 P2P message history gossip

After hello verification, the connecting peer sends history_request to
the first peer it meets (one per room, no fan-out). The responder queries
SQLite and replies with a history_chunk. Received history is stored via
INSERT OR IGNORE (mid dedup) and emitted as history_loaded IPC events.

- proto: MsgHistoryRequest/Chunk types, HistoryEntry, EvtHistoryLoaded,
  ComputeMsgID (sha256 content-addressed ID), MsgID field on ChatMessage
- store: ALTER TABLE ADD COLUMN msg_id + unique index migration (idempotent);
  RecentMessagesSince query (msg_id IS NOT NULL filter); msg_id persisted on save
- mesh: RequestHistoryFrom, HandleHistoryRequest, HandleHistoryChunk methods;
  historyRequested/historyFirstPeer state to ensure single-peer requests
- peer: dispatch history_request/history_chunk; RequestHistoryFrom after hello;
  stamp MsgID on incoming chat messages
- ipc: stamp MsgID on outgoing group chat messages
- EXTENSIONS.md: EXT-007 documented

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

internal/store/store.go

@@ -5,6 +5,7 @@ package store
 import (
 	"database/sql"
 	"fmt"
+	"strings"
 	"time"
 
 	_ "modernc.org/sqlite"
@@ -35,6 +36,14 @@ CREATE TABLE IF NOT EXISTS rooms (
 );
 `
 
+// migrations run after the base schema. ALTER TABLE ADD COLUMN fails with
+// "duplicate column name" on subsequent opens — we swallow that error.
+var migrations = []string{
+	// EXT-007: canonical message ID for history dedup (NULL for pre-feature messages).
+	`ALTER TABLE messages ADD COLUMN msg_id TEXT`,
+	`CREATE UNIQUE INDEX IF NOT EXISTS idx_messages_msg_id ON messages (msg_id) WHERE msg_id IS NOT NULL`,
+}
+
 // Store is a local SQLite-backed message and peer store.
 type Store struct {
 	db *sql.DB
@@ -51,6 +60,12 @@ func Open(path string) (*Store, error) {
 		db.Close()
 		return nil, fmt.Errorf("migrate db: %w", err)
 	}
+	for _, m := range migrations {
+		if _, err := db.Exec(m); err != nil && !strings.Contains(err.Error(), "duplicate column") {
+			db.Close()
+			return nil, fmt.Errorf("migration %q: %w", m, err)
+		}
+	}
 	return &Store{db: db}, nil
 }
 
@@ -64,9 +79,9 @@ func (s *Store) Close() error {
 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.Text, sentAt,
+		`INSERT OR IGNORE INTO messages (mid, msg_id, room, from_peer, body, sent_at)
+		 VALUES (?, ?, ?, ?, ?, ?)`,
+		msg.Mid, nullableString(msg.MsgID), msg.Room, string(msg.From), msg.Text, sentAt,
 	)
 	return err
 }
@@ -91,14 +106,31 @@ func (s *Store) PeerAlias(peerID proto.PeerID) string {
 
 // RecentMessages returns up to limit messages for a room, oldest first.
 func (s *Store) RecentMessages(room string, limit int) ([]proto.ChatMessage, error) {
-	rows, err := s.db.Query(
-		`SELECT mid, from_peer, body, sent_at
-		 FROM messages
+	return s.queryMessages(
+		`SELECT mid, from_peer, body, sent_at FROM messages
 		 WHERE room = ?
-		 ORDER BY sent_at DESC
-		 LIMIT ?`,
+		 ORDER BY sent_at DESC LIMIT ?`,
 		room, limit,
 	)
+}
+
+// RecentMessagesSince returns up to limit messages for a room with ts > sinceMs, oldest first.
+// Only messages that have a msg_id (i.e. gossip-safe) are returned.
+func (s *Store) RecentMessagesSince(room string, sinceMs int64, limit int) ([]proto.ChatMessage, error) {
+	if limit <= 0 || limit > 500 {
+		limit = 500
+	}
+	since := time.UnixMilli(sinceMs).UTC()
+	return s.queryMessages(
+		`SELECT mid, from_peer, body, sent_at FROM messages
+		 WHERE room = ? AND sent_at > ? AND msg_id IS NOT NULL
+		 ORDER BY sent_at DESC LIMIT ?`,
+		room, since, limit,
+	)
+}
+
+func (s *Store) queryMessages(q string, args ...any) ([]proto.ChatMessage, error) {
+	rows, err := s.db.Query(q, args...)
 	if err != nil {
 		return nil, err
 	}
@@ -113,7 +145,6 @@ func (s *Store) RecentMessages(room string, limit int) ([]proto.ChatMessage, err
 			return nil, err
 		}
 		m.From = proto.PeerID(from)
-		m.Room = room
 		m.Ts = sentAt.UnixMilli()
 		msgs = append(msgs, m)
 	}
@@ -168,3 +199,10 @@ func (s *Store) KnownPeers() (map[proto.PeerID]string, error) {
 	}
 	return out, rows.Err()
 }
+
+func nullableString(s string) any {
+	if s == "" {
+		return nil
+	}
+	return s
+}