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-tui/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    
+}