Fix Windows serial hangs and WinError 31 crashes

[af-t]

• Skip slow comports() scan when an explicit port is given, since the Preloader only stays alive ~0.3s
• Set write_timeout so a stuck write can't block forever uninterruptibly, and chunk usbwrite() at the packet size so timeout headroom is real per chunk; abort instead of retrying from a stale position if a write genuinely times out, since pyserial can't say how many bytes already went out
• Bound flush() instead of trusting the driver's unreliable out_waiting/in_waiting reporting, which otherwise hangs indefinitely
• Read DAXML frames staged with explicit lengths instead of one guessed read, fixing both premature "Wrong length" and framing corruption ("Wrong magic"); give the sub-reads real slack against driver jitter
• Only reassign device.timeout when it actually changes, and never reassign it in the resplen=None path, to avoid WinError 31 right after a write burst; lower the constructor default so that path's wait bound is real instead of relying on a stale ambient value
• Propagate serialportname onto Port so reconnects keep the same port
• Fix mtk.py argparse: global options (--vid, --stock, etc.) given before the subcommand were silently reset to their defaults, since every subcommand parser redeclares them via parents=[base] and argparse's subparser dispatch overwrites the parent namespace with the subparser's own defaults