Replace libtelnet with MTHClient and add truecolor ANSI parsing

[powerje]

Summary

• Replace vendored libtelnet C library with MTHClient Swift package for telnet protocol handling
• Rewrite SSMUDSocket in Swift, delegating telnet negotiation to MTHClient
• Add truecolor (24-bit) ANSI color parsing support in SSANSIEngine
• Remove SPLTelnetLib Objective-C wrapper and associated tests
• Add local/remote package resolution for MTHClient (local for dev, tagged release for CI)

Test plan

• Verify MUD connections work end-to-end (connect, send commands, receive output)
• Verify ANSI colors render correctly including 256-color and truecolor
• Verify telnet negotiation (NAWS, MCCP2) works with MUD servers
• Run MRTests test suite