Migrate client to Kotlin/JVM with Java 8 runtime compatibility

[gradwahl]

Goal

Convert the client from Java to Kotlin/JVM while preserving compatibility with a Java 8 runtime.

First migration batch

• Add Kotlin Maven plugin and Kotlin stdlib.
• Compile Kotlin sources with jvmTarget = 1.8 (Java 8 class-file version 52).
• Convert IsaacCipher, PacketBuffer, Protocol, and LoginResult to Kotlin.
• Preserve Java-facing fields, static constants, constructors, and record-style accessors so remaining Java sources can interoperate during the migration.

Validation performed

• Kotlin classes compile with JVM target 1.8.
• javap reports class-file major version 52.
• A Java interop harness compiled with --release 8 successfully accesses the migrated API.
• 2,000 ISAAC outputs match the original Java implementation.

Important migration state

This PR is intentionally a draft. The repository is not yet fully Java 8 runnable because remaining Java files still contain Java 17-era syntax and Maven must temporarily compile those Java sources as Java 17. The final migration step will switch the Java compiler target to 8 (or remove Java compilation entirely) after the remaining .java files are converted and Java 9+ API usages are removed/replaced.

Next batches

Convert the remaining packages incrementally, prioritising Java 17-only constructs (record, switch expressions, etc.), then run a full Java 8 compatibility pass over library/API usage and packaging.