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)

.gitignore

@@ -4,6 +4,9 @@
 debug/
 target/
 
+# Internal docs (not for end users)
+docs/
+
 # These are backup files generated by rustfmt
 **/*.rs.bk