feat: add FFI layer, protocol tests, mock transport, README
- FFI: eskin_open/close/read_register/write_register for C/C++/Python - Protocol: encode/decode tests with golden bytes verification - Stream: implement PollingSampleCollector producing FingerSample - Register: add parse_combined_forces/parse_module_errors - Transport: add MockSerialTransport for testing - Include: add C header file eskin_ffi.h - Examples: C++ and Python usage examples - README: full usage guide for Rust/C++/Python - Exclude docs/ from repo (internal only)
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