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/mesh/mesh.go

@@ -2,6 +2,7 @@
 package mesh
 
 import (
+	"encoding/json"
 	"log"
 	"os"
 	"sync"
@@ -51,6 +52,12 @@ type Mesh struct {
 	// attempt to connect to. Drained by the anchor client's runOnce loop.
 	PendingConnect chan proto.PeerID
 
+	// historyRequested tracks rooms for which we have already sent a history_request
+	// this session. Reset on reconnect is intentional (new peers may have newer history).
+	historyMu        sync.Mutex
+	historyRequested map[string]bool // room → true
+	historyFirstPeer proto.PeerID    // ID of the peer we requested history from
+
 	// subscribers receive a copy of every event (fan-out to IPC clients)
 	subMu sync.Mutex
 	subs  []chan proto.IpcMessage
@@ -60,12 +67,13 @@ type Mesh struct {
 // Pass a non-nil store to enable message and peer persistence.
 func New(id *crypto.Identity, st *store.Store) *Mesh {
 	return &Mesh{
-		Identity:       id,
-		Store:          st,
-		peers:          make(map[proto.PeerID]*PeerConn),
-		outbound:       make(map[string]*outboundTransfer),
-		inbound:        make(map[string]*inboundTransfer),
-		PendingConnect: make(chan proto.PeerID, 32),
+		Identity:         id,
+		Store:            st,
+		peers:            make(map[proto.PeerID]*PeerConn),
+		outbound:         make(map[string]*outboundTransfer),
+		inbound:          make(map[string]*inboundTransfer),
+		PendingConnect:   make(chan proto.PeerID, 32),
+		historyRequested: make(map[string]bool),
 	}
 }
 
@@ -235,6 +243,122 @@ func (m *Mesh) Unsubscribe(ch <-chan proto.IpcMessage) {
 	}
 }
 
+// RequestHistoryFrom sends history_request messages to peerID for all rooms
+// we know about but haven't yet requested this session. Only contacts the first
+// peer we connect to, to avoid fan-out amplification.
+func (m *Mesh) RequestHistoryFrom(peerID proto.PeerID) {
+	if m.Store == nil {
+		return
+	}
+	m.historyMu.Lock()
+	if m.historyFirstPeer != "" && m.historyFirstPeer != peerID {
+		m.historyMu.Unlock()
+		return // only request from the first peer
+	}
+	m.historyFirstPeer = peerID
+	m.historyMu.Unlock()
+
+	rooms, err := m.Store.Rooms()
+	if err != nil {
+		return
+	}
+	// Always include "general" even if not explicitly created.
+	roomSet := map[string]bool{"general": true}
+	for _, r := range rooms {
+		roomSet[r] = true
+	}
+
+	m.historyMu.Lock()
+	var toRequest []string
+	for r := range roomSet {
+		if !m.historyRequested[r] {
+			m.historyRequested[r] = true
+			toRequest = append(toRequest, r)
+		}
+	}
+	m.historyMu.Unlock()
+
+	for _, room := range toRequest {
+		req, err := json.Marshal(proto.PeerMessage{
+			Type:  proto.MsgHistoryRequest,
+			Room:  room,
+			Limit: 200,
+		})
+		if err != nil {
+			continue
+		}
+		m.SendTo(peerID, req)
+		log.Printf("mesh: sent history_request room=%s to %s", room, peerID.Short())
+	}
+}
+
+// HandleHistoryRequest responds to a history_request from a peer.
+func (m *Mesh) HandleHistoryRequest(from proto.PeerID, room string, sinceMs int64, limit int) {
+	if m.Store == nil {
+		return
+	}
+	msgs, err := m.Store.RecentMessagesSince(room, sinceMs, limit)
+	if err != nil {
+		log.Printf("mesh: history_request from %s room=%s: %v", from.Short(), room, err)
+		return
+	}
+
+	// Look up aliases for from_peer values.
+	entries := make([]proto.HistoryEntry, 0, len(msgs))
+	for _, msg := range msgs {
+		entries = append(entries, proto.HistoryEntry{
+			Mid:       msg.Mid,
+			From:      string(msg.From),
+			FromAlias: m.Store.PeerAlias(msg.From),
+			Text:      msg.Text,
+			Ts:        msg.Ts,
+		})
+	}
+
+	chunk, err := json.Marshal(proto.PeerMessage{
+		Type:        proto.MsgHistoryChunk,
+		Room:        room,
+		History:     entries,
+		HistoryDone: true,
+	})
+	if err != nil {
+		return
+	}
+	m.SendTo(from, chunk)
+	log.Printf("mesh: sent history_chunk room=%s to %s: %d msgs", room, from.Short(), len(entries))
+}
+
+// HandleHistoryChunk saves received history messages and emits history_loaded.
+func (m *Mesh) HandleHistoryChunk(room string, entries []proto.HistoryEntry) {
+	if m.Store == nil || len(entries) == 0 {
+		return
+	}
+	var saved []proto.ChatMessage
+	for _, e := range entries {
+		msg := &proto.ChatMessage{
+			Mid:   e.Mid,
+			MsgID: e.Mid, // mid is already content-addressed for gossipped messages
+			From:  proto.PeerID(e.From),
+			Room:  room,
+			Text:  e.Text,
+			Ts:    e.Ts,
+		}
+		if err := m.Store.SaveMessage(msg); err != nil {
+			continue
+		}
+		saved = append(saved, *msg)
+	}
+	if len(saved) == 0 {
+		return
+	}
+	m.emit(proto.IpcMessage{
+		Type:     proto.EvtHistoryLoaded,
+		Room:     room,
+		Messages: saved,
+	})
+	log.Printf("mesh: history_chunk room=%s: %d/%d new messages", room, len(saved), len(entries))
+}
+
 // Emit sends an event to all IPC subscribers (exported for ipc/nat packages).
 func (m *Mesh) Emit(msg proto.IpcMessage) {
 	m.emit(msg)