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/lib.rs

@@ -0,0 +1,7 @@
+pub mod error;
+pub mod transport;
+pub mod protocol;
+pub mod frame;
+pub mod pipeline;
+
+pub use error::{Error, Result};