Add rcon exec and rcon eval commands

[sakuro]

Summary

Add factorix rcon exec and factorix rcon eval commands using the rcon-client gem to control a running Factorio server via RCON.

Commands

• factorix rcon exec <command> — send a Factorio console command (e.g. /server-save)
• factorix rcon eval <script> — evaluate a Lua script (e.g. rcon.print(game.tick))

Details

• Add rcon-client gem dependency
• Add setting :rcon block to Factorix config (host, port, password); each command accepts --host, --port, --password options to override config values
• rcon eval wraps the script in /c <script> before sending
• RCon::Client must be opened with sentinel_command: "/c" — Factorio does not respond to an empty sentinel, which would cause execute to hang
• Map RCon::Client::ConnectionError / AuthenticationError to a new Factorix::RCONError hierarchy under InfrastructureError

New error classes

RCONError < InfrastructureError
  RCONConnectionError < RCONError
  RCONAuthenticationError < RCONError