Add Kotlin port with server and client telnet sessions

[powerje]

• Adds a complete Kotlin/JVM port of MTH under kotlin/, structured as a Gradle multi-module project (mth-core, mth-color)
• Server-side telnet session (mth.core.server) ported from the Swift implementation
• New client-side telnet session (mth.core.client) for use in MUD clients like Wamdroid — handles the inverse negotiation direction (receives WILL/DO from server, responds DO/WILL)
• Shared types (TelnetConstants, Compression, MSDP, CommFlags, MTTSFlags) in mth.core
• GitHub Actions CI for Kotlin (Ubuntu, JDK 17, Gradle 9.4)
• README updated with Kotlin CI badge, client/server usage examples, and Gradle import instructions