Replace blocking ioctl(SG_IO) with the sg driver's async interface.
Commands are submitted via write(), waited on via poll() with a hard
wall-clock timeout, and completed via read(). If poll() times out,
the fd is abandoned and a fresh one opened — the kernel can no longer
hold us hostage during USB error recovery.
- write() submits command, returns immediately
- poll() enforces exact timeout (EINTR-safe with deadline tracking)
- read() retrieves result + copies data to caller's buffer
- On timeout: old fd closed in background thread, new fd opened
- No SG_FLAG_DIRECT_IO — kernel buffers for safe timeout abandonment
- Store device_path for fd reopen after timeout
- Drop guards fd=-1 (abandoned fd)
Matt Jackson
71e1f57364