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)

include/eskin_ffi.h

@@ -0,0 +1,67 @@
+#ifndef ESkin_FFI_H
+#define ESkin_FFI_H
+
+#include <cstdint>
+#include <stdint.h>
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef void* EskinDeviceHandle;
+
+typedef struct {
+    uint16_t major;
+    uint16_t minor;
+    uint16_t patch;
+} EskinSdkVersion;
+
+typedef enum {
+    ESkinSuccess = 0,
+    ESkinInvalidPointer = 1,
+    ESkinDeviceNotFound = 2,
+    ESkinDeviceAlreadyOpen = 3,
+    ESkinNotInitialized = 4,
+    ESkinAlreadyStreaming = 5,
+    ESkinNotStreaming = 6,
+    ESkinConfigError = 7,
+    ESkinIoError = 8,
+    ESkinTimeout = 9,
+    ESkinChannelClosed = 10,
+    ESkinInternalError = 11,
+    ESkinBufferOverflow = 12,
+    ESkinInvalidParameter = 13,
+    ESkinCrcError = 14,
+    ESkinFrameError = 15,
+    ESkinProtocolError = 16,
+    ESkinDeviceError = 17,
+} EskinSdkErrorCode;
+
+EskinSdkVersion eskin_version(void);
+
+EskinDeviceHandle eskin_open(const char* path, const void* config);
+EskinSdkErrorCode eskin_close(EskinDeviceHandle handle);
+
+EskinSdkErrorCode eskin_read_register(
+    EskinDeviceHandle handle,
+    uint32_t addr,
+    uint16_t length,
+    uint8_t* buf,
+    uint32_t buf_len,
+    uint32_t* actual_len
+);
+
+EskinSdkErrorCode eskin_write_register(
+    EskinDeviceHandle handle,
+    uint32_t addr,
+    const uint8_t* data,
+    uint16_t data_len,
+    uint16_t* return_count
+);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif