feat: initial xserial-core with transport, protocol, frame, and pipeline layers

- transport: async Serial/TCP/UDP via Connection enum + Transport trait
- protocol: TextDecoder (UTF-8/Latin-1/ASCII), HexDecoder, PlotDecoder (binary samples)
- frame: LineFramer, FixedLengthFramer, LengthPrefixedFramer, CobsFramer
- pipeline: MultiPipeline for parallel multi-protocol parsing
- 212 tests (182 unit + 18 integration + 1 doctest), clippy clean

crates/xserial-core/src/error.rs

@@ -0,0 +1,27 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("I/O error: {0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("Serial port error: {0}")]
+    Serial(#[from] serialport::Error),
+
+    #[error("Transport not connected")]
+    NotConnected,
+
+    #[error("Connection failed: {0}")]
+    ConnectionFailed(String),
+
+    #[error("Unsupported operation: {0}")]
+    Unsupported(String),
+
+    #[error("Protocol error: {0}")]
+    Protocol(String),
+
+    #[error("Script error: {0}")]
+    Script(String),
+}
+
+pub type Result<T> = std::result::Result<T, Error>;