Serialize transport access for async calls

[c3rebro]

Motivation

• Concurrent async calls could interleave writes/reads on the underlying COM transport and provoke device-level AccessDenied errors, so transport access needs serialization inside the library.

Description

• Added an async lock field SemaphoreSlim _transportLock to TWN4ReaderDevice to serialize access to the transport.
• Wrapped the transport TX/RX path in DoTXRXAsync with await _transportLock.WaitAsync() and a finally that calls _transportLock.Release() to ensure calls do not overlap.
• Added a test helper ConcurrentReaderTransport that detects overlapping write/read cycles.
• Added a unit test CallFunctionRawAsync_SerializesConcurrentCalls to verify concurrent CallFunctionRawAsync calls are serialized and do not overlap.

Testing

• Ran dotnet test tests/Elatec.NET.Tests/Elatec.NET.Tests.csproj, which executed the test suite and passed: 24 passed, 0 failed.

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Codex Task