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)

src/types.rs

@@ -54,6 +54,43 @@ pub enum SensorModule {
     Palm8 = 27,
 }
 
+impl SensorModule {
+    pub fn from_index(index: u8) -> Self {
+        match index {
+            0 => Self::ThumbProximal,
+            1 => Self::ThumbMiddle,
+            2 => Self::ThumbTip,
+            3 => Self::ThumbNail,
+            4 => Self::IndexProximal,
+            5 => Self::IndexMiddle,
+            6 => Self::IndexTip,
+            7 => Self::IndexNail,
+            8 => Self::MiddleProximal,
+            9 => Self::MiddleMiddle,
+            10 => Self::MiddleTip,
+            11 => Self::MiddleNail,
+            12 => Self::RingProximal,
+            13 => Self::RingMiddle,
+            14 => Self::RingTip,
+            15 => Self::RingNail,
+            16 => Self::PinkyProximal,
+            17 => Self::PinkyMiddle,
+            18 => Self::PinkyTip,
+            19 => Self::PinkyNail,
+            20 => Self::Palm1,
+            21 => Self::Palm2,
+            22 => Self::Palm3,
+            23 => Self::Palm4,
+            24 => Self::Palm5,
+            25 => Self::Palm6,
+            26 => Self::Palm7,
+            27 => Self::Palm8,
+            _ => Self::ThumbProximal,
+        }
+    }
+}
+
+
 pub const SENSOR_MODULE_COUNT: usize = 28;
 
 #[repr(C)]