Add Windows support and native PlatformIO test coverage

[yanxke]

Summary

Adds Windows support to the desktop Arduino emulator while preserving the existing POSIX/Linux implementation. Tested on both Windows and Ubuntu.

Changes

• Fix RingBufferExt handling of valid 0x00 payload bytes.
• Add platform detection and cross-platform socket abstractions.
• Implement Windows Winsock support.
• Split POSIX and Windows Ethernet server implementations.
• Make UDP networking cross-platform and correctly handle hostname resolution, non-blocking reads, socket errors, and send/receive loopback traffic.
• Add Windows serial support, including COM ports 10 and above.
• Make physical serial devices opt-in when running on Windows.
• Add Windows-compatible SD directory creation.
• Add a native PlatformIO example and Unity test suite.

Testing

Verified with:
pio test -e native
All 9 native tests passed, including GPIO delegation, SD filesystem operations, UDP loopback, TCP loopback, socket error mapping, Arduino types, and timing behavior.

[pschatzmann]

Review of PR #24 found 4 issues, ranked by severity:

• entry.cpp:20 — Windows WinMain calls setup()/loop() directly, skipping hardwareSetup() that the POSIX path runs via Main.cpp. On Windows builds with USE_RPI/USE_FTDI/USE_REMOTE, hardware peripherals never get initialized.
• SocketImplWindows.cpp:1184 — getIPAddress(validEntries[]) on Windows ignores the interface filter entirely, always returning the hostname's address instead of matching POSIX's interface-selection behavior.
• EthernetServerWindows.h:725 — Windows begin_() swallows listen failures silently (no Logger.error), unlike POSIX which logs "listen failed" — makes port-in-use failures hard to diagnose.
  Simplification (1):
• . Serial.h:36 — The new #if ARDUINO_EMULATOR_WINDOWS/#else branches are identical, dead conditional code that risks silent divergence on future edits.

[yanxke]

Review of PR #24 found 4 issues, ranked by severity:

• entry.cpp:20 — Windows WinMain calls setup()/loop() directly, skipping hardwareSetup() that the POSIX path runs via Main.cpp. On Windows builds with USE_RPI/USE_FTDI/USE_REMOTE, hardware peripherals never get initialized.
• SocketImplWindows.cpp:1184 — getIPAddress(validEntries[]) on Windows ignores the interface filter entirely, always returning the hostname's address instead of matching POSIX's interface-selection behavior.
• EthernetServerWindows.h:725 — Windows begin_() swallows listen failures silently (no Logger.error), unlike POSIX which logs "listen failed" — makes port-in-use failures hard to diagnose.
  Simplification (1):
• . Serial.h:36 — The new #if ARDUINO_EMULATOR_WINDOWS/#else branches are identical, dead conditional code that risks silent divergence on future edits.

All addressed.