feat: xserial-client with session management, mlua integration, and 238 tests

- Session: background I/O task with SessionHandle for send/read/close/reconfigure
- SessionManager: multi-session collection with aggregated event broadcast
- Config: serializable SessionConfig/PipelineConfig/FramerConfig/DecoderConfig
- Lua: xserial global (open/list_ports/sleep/log) + session userdata API
- RingBuffer: bounded history buffer with pipeline filtering
- Core: added Serialize/Deserialize+Default to Endian/TextEncoding/PlotFormat
- Core: added Send bound to Framer trait for tokio task compatibility
- 238 tests (193 core + 10 config + 7 Lua + 9 session + 18 pipeline + 1 doctest)

crates/xserial-client/src/error.rs

@@ -0,0 +1,17 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("Connection failed: {0}")]
+    ConnectionFailed(String),
+    #[error("Session error: {0}")]
+    Session(String),
+    #[error("Lua error: {0}")]
+    Lua(#[from] mlua::Error),
+    #[error("I/O error: {0}")]
+    Io(#[from] std::io::Error),
+    #[error("{0}")]
+    Other(String),
+}
+
+pub type Result<T> = std::result::Result<T, Error>;