Add SQLite message and peer persistence (internal/store)

Each daemon writes to <data-dir>/messages.db on startup. Messages received
or sent are stored immediately; duplicate mids (INSERT OR IGNORE) are safe
to call multiple times. Peer aliases are upserted on peer_connected and
again after hello verification when the real nick is known.

Schema
- messages(mid UNIQUE, room, from_peer, body, sent_at) — mid is the YAW/2
  dedup key added in the proto migration; index on (room, sent_at) for
  efficient per-room queries.
- peers(peer_id PK, alias, last_seen) — cache of every peer ever seen,
  used to resolve hex ids to names when peers are offline.

Wiring
- store.Open called in cmd/daemon/main.go, passed to mesh.New.
- mesh.Mesh holds *store.Store (nil-safe; persistence is optional).
- mesh.SaveMessage called in dispatchPeerMessage (incoming) and ipc
  CmdSendMessage (outgoing) so the local node's own messages are stored.
- mesh.UpdatePeerAlias called after hello verification updates the alias
  with the verified nick rather than the placeholder short-id.

Messages only accumulate from join time forward — no history replay to
late-joining peers; each node's view starts from when it connected.

test-network.sh: added SQLite verification block that queries each node's
DB after the test and prints message + peer counts.

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

test-network.sh

@@ -244,6 +244,21 @@ echo -e "${DIM}─────────────────────
 ipc "$CHARLIE_IPC" '{"type":"leave_network"}'
 sleep 1
 
+echo ""
+echo -e "${DIM}────────────────────────────────────────────────────────${RESET}"
+echo -e "verifying persistence (SQLite)"
+echo -e "${DIM}────────────────────────────────────────────────────────${RESET}"
+for peer in alice bob charlie; do
+    db="$DATA_ROOT/$peer/messages.db"
+    if [ -f "$db" ]; then
+        count=$(sqlite3 "$db" "SELECT COUNT(*) FROM messages;" 2>/dev/null || echo "?")
+        peers=$(sqlite3 "$db" "SELECT COUNT(*) FROM peers;" 2>/dev/null || echo "?")
+        echo -e "  ${BOLD}${peer}${RESET}: ${count} messages, ${peers} known peers in messages.db"
+    else
+        echo -e "  ${RED}${peer}: no messages.db found${RESET}"
+    fi
+done
+
 echo ""
 echo -e "${DIM}────────────────────────────────────────────────────────${RESET}"
 echo -e "${GREEN}${BOLD}test complete${RESET}"