cmd/tailcat: serve Unix-domain sockets

[atirna]

Summary

• Add tailcat serve --unix-socket=<path> for a local Unix-domain stream socket.
• Each inbound tailcat stream gets a fresh connection to that socket on port 1, leaving the socket's existing filesystem permissions unchanged.

Why

This lets a local Unix-socket service be reached directly without first exposing it on a TCP port.

Testing

• Before: current main has no --unix-socket serve option.
• After: go test ./cmd/tailcat -run '^TestServeUnixSocket$' -count=1

Fixes #93

[atirna]

added: --unix-socket now takes an optional ,port suffix (port 1 stays the default), the packet filter and dispatch key on the parsed port, and the README shows the forward command for reaching a custom port. ,8022 with an empty pathname is rejected at startup.