Replace etcp_router_bind/etcp_route_send/etcp_router_unbind with
direct etcp_bind/etcp_send/etcp_unbind + find-conn-by-node_id helpers.
Same approach as db_sync (0x20). Service IDs unchanged (0x30, 0x31).
- Replace get_time_us() with utun_gettimeofday() for PUSH message timestamps
(db_sync.c, chat_core.c) — monotonic clocks differ across Linux/Windows
- Fix merkle_sync: restart session timer in _handle_request to prevent
double-sided timeout when both nodes start member_sync simultaneously
- Fix INIT_RESP short-format: don't mark synced when my_count > peer_count
- Add db_sync_get_last_timestamp() to avoid duplicate timestamp generation
between db_sync and chat_core on_db_sync_insert
- Fix db_sync_stub to generate timestamps consistently
- build.sh: auto-rebuild chatgui via CMake after utun build
Evgeny