Harden serial driver I/O behavior and request safety

[coxep]

Summary

• harden Interface transaction behavior to handle partial serial writes and reads reliably
• enforce request-direction safety at runtime (read on write-only and write on read-only now throw std::invalid_argument)
• harden SerialDevice low-level I/O behavior and serial configuration:
  • return actual bytes written from write (fixing bool-as-size_t bug)
  • handle EINTR and nonblocking EAGAIN/EWOULDBLOCK with retry
  • use correct select nfds (fd + 1) and timeout handling
  • add configurable read timeout and baud rate setters
  • configure termios with raw mode, explicit baud rate, and VMIN/VTIME settings
• add regression tests proving:
  • write path retries until full payload is transferred
  • read path retries until full fixed-size response is assembled
  • unsupported request direction operations fail fast with std::invalid_argument

Testing

• cmake -DCMAKE_CXX_COMPILER=g++ ..
• cmake --build . -j
• ctest --output-on-failure

All tests pass (6/6).

Notes

• branch includes explicit TDD red-phase commit followed by green implementation commit.