Add FFI cdylib for OSC IR message handling

[Nagitch]

Summary

• add the new osc-ffi crate and wire it into the workspace so a cdylib is produced
• implement the OscMessage handle, serialization helpers, and safe buffer management for the FFI API
• include smoke tests that exercise handle creation, argument addition, msgpack serialization, and cleanup

Testing

• cargo test -p osc-ffi

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Codex Task