PlatformIO support and add more compatibility

[yanxke]

• Add PlatformIO native-platform library metadata and a top-level emulator entry point.
• Add the legacy wiring.h compatibility header and required include paths.
• Improve Serial input on POSIX systems with non-blocking buffered reads, and make global serial instances safe across translation units.
• Extend SD emulation with media presence and storage-size APIs backed by the current working directory instead of hard coded /home.
• Make millis() and micros() use a monotonic clock with Arduino-style elapsed-time semantics.