- New SVC_ROUTE_HDR (packed struct, 22 bytes): cmd+dst+src+seq+svc_id
- ETCP_ROUTER_CONN: state per (remote_node_id, svc_id), hash-indexed in router_conns
- Reorder via recv_q (hash by seq), dedup with 32-bit circular compare
- Periodic ACK (100ms), idle ACK (500ms), inflight limit via tx_seq - rx_acked
- Legacy mode: no conn → direct delivery without reorder
- etcp_router_conn_get/send/close API for seq-managed connections
- Unit test test_etcp_router_unit: 18 tests, 3ms, no ETCP/sockets/BGP