Add oxbitnet-java crate — Java/JNI bindings

[m96-chan]

Summary

• Add oxbitnet-java crate with JNI native methods wrapping oxbitnet directly (same pattern as oxbitnet-python with PyO3)
• Java API: BitNet.loadSync(), generate(), chat() with TokenCallback streaming, AutoCloseable support
• Includes Java source files (io.github.m96chan.oxbitnet package), Gradle build stub, Chat.java example, and crate README

Closes #5

Details

Rust side (src/lib.rs):

• JavaBitNet struct owns Option<BitNet> + tokio::Runtime (same as Python)
• JNI functions: nativeLoad, nativeGenerate, nativeChat, nativeFree
• Progress callback via GlobalRef + JavaVM::attach_current_thread
• Token callback: boolean onToken(String) — return false to stop early
• Errors thrown as OxBitNetException via env.throw_new()

Java side (7 files):

• BitNet — main class, AutoCloseable, stores long nativeHandle
• ChatMessage — record (role, content)
• GenerateOptions — builder pattern (maxTokens, temperature, topK, repeatPenalty, repeatLastN)
• TokenCallback — @FunctionalInterface
• LoadOptions — progress callback + cache dir
• LoadProgress — record for progress info
• OxBitNetException — RuntimeException subclass

Test plan

• cargo build -p oxbitnet-java compiles to .so
• cargo clippy -p oxbitnet-java -- -D warnings passes clean
• Compile and run Chat.java example with a GGUF model
• Verify: loads model, streams tokens via callback, clean shutdown

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