feat(cli): identify serial ports and add discover options

Proposal

Make instro discover a renderer over instro.lib.discover.discover() and expose its knobs:

--serial usb|all|none (default usb): probe only USB serial adapters, every port, or none.
Serial settings for the one probe every serial port gets (library default is 9600 baud, 8N1, no flow control): --baud N, --data-bits 7|8, --stop-bits 1|1.5|2, --parity N|O|E|M|S, --flow-control none|xon-xoff|rts-cts|dtr-dsr. These build the SerialConfig passed as discover(serial_config=...); there is no sweep, so a port that does not answer stays in the serial table with the "set serial_config manually" hint, and the user re-runs with the right flags. The hint text should name these flags.
--timeout SECONDS per-probe timeout.
--exclude RESOURCE (repeatable) to skip ports that must not be probed.
Positional RESOURCE... extra addresses VISA does not enumerate (e.g. TCPIP0::10.0.0.5::5025::SOCKET).
Per-probe progress line driven by on_event.
Serial table shows identity, driver and the serial settings that answered; ports that did not answer keep the manual-settings hint.

Update docs/cli.md. Depends on the serial identification / providers issue.