Nonblocking serial I/O

SerialIo provides try_read and try_write over caller-owned byte slices. Calls do one bounded attempt without waiting, locking or allocating. Zero means no progress; disconnected devices report an error. Writes may accept a prefix, whose length is returned. Implementations must return a count within the slice.

ReadySerial<S> adapts embedded-io Read + Write + ReadReady + WriteReady HALs. It tests readiness and makes at most one read/write call, preserving the HAL's nonblocking guarantee. HALs with DMA or different APIs can implement SerialIo directly. Linux bundles can export LinuxNonblockingSerialPort using the explicit serialN_nonblocking RON setting.