Add Haskell bindings for oxbitnet (Issue #8)

[m96-chan]

Summary

• Add Haskell bindings wrapping oxbitnet-ffi via hsc2hs for correct cross-platform struct layout
• High-level API (OxBitNet module) with MVar + bracket for thread-safe resource management, IO Bool token callbacks, and typed exceptions
• Low-level FFI module (Foreign.OxBitNet.Raw) with Storable instances, callback wrappers, and safe/unsafe FFI imports
• Interactive chat REPL example (oxbitnet-chat)
• CI: version check for oxbitnet.cabal + publish-hackage job (GHC 9.6, Cabal 3.10, source-only Hackage upload)

Design decisions

• MVar (Maybe (Ptr OxBitNet)) instead of ForeignPtr — oxbitnet_free blocks (tokio runtime shutdown), unsafe on GC finalizer thread
• safe FFI for load/generate/chat (long-running + callbacks); unsafe for free/error_message/defaults
• Token callback returns IO Bool (True = stop) — more idiomatic Haskell than Int32
• Logger FunPtr intentionally leaked (process-lifetime, matching C API contract)
• Source-only Hackage distribution — user links against their local liboxbitnet_ffi

Test plan

• cargo build -p oxbitnet-ffi --release then cabal build all
• Run oxbitnet-chat with a GGUF model, verify streaming output + progress
• Verify freeBitNet / bracket cleanup
• Verify CI version check parses oxbitnet.cabal correctly

Closes #8

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