Harden server network exposure defaults

[AlexJuca]

This PR is part of #77 and does the following:

• bind the TCP server to 127.0.0.1 by default and add an explicit bind server config option
• add max-clients with a conservative default of 128 and use it for the listen backlog
• reject accepted sockets over capacity before allocating client state or queuing reads in kqueue, epoll, and io_uring paths
• replace permissive atoi parsing for server port, event-loop max events, and max-clients with bounded decimal parsing
• add config and server-limit unit tests

Notes

This PR intentionally does not close #77 because idle/slow-client cleanup remains as a follow-up slice.