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
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