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-client/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "xserial-client"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+xserial-core = { path = "../xserial-core" }
+tokio = { workspace = true }
+tracing = { workspace = true }