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/proto/proto.go

@@ -3,7 +3,11 @@
 // Binary data (keys, signatures) is hex-encoded; signaling boxes are base64.
 package proto
 
-import "time"
+import (
+	"crypto/sha256"
+	"fmt"
+	"time"
+)
 
 // ── Identity ──────────────────────────────────────────────────────────────────
 
@@ -43,8 +47,10 @@ const (
 	MsgFileAccept   MsgType = "file-accept"
 	MsgFileCancel   MsgType = "file-cancel"
 	MsgFileDone     MsgType = "file-done"
-	MsgPing         MsgType = "ping"
-	MsgPong         MsgType = "pong"
+	MsgPing            MsgType = "ping"
+	MsgPong            MsgType = "pong"
+	MsgHistoryRequest  MsgType = "history_request"
+	MsgHistoryChunk    MsgType = "history_chunk"
 )
 
 // PmMessage is a private message sent directly over a single peer link (§8 "pm").
@@ -83,17 +89,43 @@ type PeerMessage struct {
 	Reason string `json:"reason,omitempty"` // file-cancel
 
 	Seq *uint64 `json:"seq,omitempty"` // ping/pong
+
+	// history_request fields
+	Since int64 `json:"since,omitempty"` // Unix ms; 0 = no lower bound
+	Limit int   `json:"limit,omitempty"`
+
+	// history_chunk fields
+	History     []HistoryEntry `json:"history,omitempty"`
+	HistoryDone bool           `json:"history_done,omitempty"`
+}
+
+// HistoryEntry is one message in a history_chunk response.
+type HistoryEntry struct {
+	Mid       string `json:"mid"`
+	From      string `json:"from"`       // peer ID hex
+	FromAlias string `json:"from_alias"` // advisory
+	Text      string `json:"text"`
+	Ts        int64  `json:"ts"` // Unix ms
 }
 
 // ChatMessage is a group chat message (wire type "chat", §8).
 // Also used internally for persisting PMs after they are received.
 type ChatMessage struct {
-	Mid  string `json:"mid,omitempty"` // optional dedup id (required when relay hops > 0)
-	From PeerID `json:"from,omitempty"` // set by receiver from DC context; not on wire for pm
-	To   *PeerID `json:"to,omitempty"`  // internal only — not transmitted; set for DMs
-	Room string `json:"room"`
-	Text string `json:"text"`
-	Ts   int64  `json:"ts"` // Unix milliseconds
+	Mid   string  `json:"mid,omitempty"`   // optional dedup id (required when relay hops > 0)
+	MsgID string  `json:"msg_id,omitempty"` // EXT-007: content-addressed gossip ID
+	From  PeerID  `json:"from,omitempty"`  // set by receiver from DC context; not on wire for pm
+	To    *PeerID `json:"to,omitempty"`    // internal only — not transmitted; set for DMs
+	Room  string  `json:"room"`
+	Text  string  `json:"text"`
+	Ts    int64   `json:"ts"` // Unix milliseconds
+}
+
+// ComputeMsgID returns the EXT-007 content-addressed ID for a message.
+// sha256(fromID \x00 room \x00 ts_decimal \x00 text)
+func ComputeMsgID(fromID PeerID, room string, ts int64, text string) string {
+	h := sha256.New()
+	fmt.Fprintf(h, "%s\x00%s\x00%d\x00%s", string(fromID), room, ts, text)
+	return fmt.Sprintf("sha256:%x", h.Sum(nil))
 }
 
 // PeerGossip shares known peer addresses.
@@ -259,6 +291,7 @@ const (
 	EvtIdentityImported IpcMsgType = "identity_imported"
 	EvtSharesList       IpcMsgType = "shares_list"
 	EvtRoomCreated      IpcMsgType = "room_created"    // field: room (name)
+	EvtHistoryLoaded    IpcMsgType = "history_loaded"  // fields: room, messages
 )
 
 // NetworkInfo summarises one joined network for state_snapshot and network_joined events.
@@ -312,7 +345,8 @@ type IpcMessage struct {
 	Networks       []NetworkInfo `json:"networks,omitempty"`
 	ErrorMessage   string       `json:"error_message,omitempty"`
 	InviteGenerated string      `json:"invite,omitempty"`
-	Files          []FileEntry  `json:"files,omitempty"`
+	Files          []FileEntry    `json:"files,omitempty"`
+	Messages       []ChatMessage  `json:"messages,omitempty"` // history_loaded
 	Shares         []ShareEntry `json:"shares,omitempty"`
 	ShareNetworks  []string     `json:"network_ids,omitempty"` // for add_share command: scope to specific network IDs, or ["*"] for global
 	// export_identity / import_identity