- Add etcp_find_free_local_link_id() function to allocate unique link IDs
- Modify etcp_link_new() to auto-assign local_link_id, fail if none available
- Update INIT_REQUEST (0x02/0x04) to send local_link_id after keepalive
- Update INIT_RESPONSE (0x03/0x05) to include local_link_id
- Parse remote_link_id from incoming handshake packets
- Update protocol documentation in doc/etcp_protocol.txt
- Add comprehensive unit test test_etcp_link_id.c
New packet format:
INIT_REQUEST: [code][node_id(8)][mtu(2)][keepalive(2)][link_id(1)][pubkey(64)]
INIT_RESPONSE: [code][node_id(8)][mtu(2)][link_id(1)]
Evgeny