fix(telemetrix): a dropped serial link reads as disconnected

[gbradham]

Problem

An experiment on an infinite Loop driving an Arduino would run for a while, then the hardware stopped responding. Nothing was logged.

When the USB serial link drops, telemetrix-aio's TelemetrixAioSerial.write() catches the SerialException, closes the port, waits one second and returns None. We pass close_loop_on_shutdown=False, so it never raises and never stops its event loop. The worker thread stays alive, TelemetrixBoard.is_connected stays True, and every later digital_write looks like it worked while nothing reaches the board.

The one thing that does fail with the port is telemetrix's report reader (the_task). It dies on the first in_waiting poll, and its exception is stored on a task that nothing ever reads, so it is never logged either.

Fix

TelemetrixThread.is_running now also requires the report reader to be alive. A reader that ended because of a deliberate shutdown() still counts as alive. A new link_error property gives the reason. is_connected already checks is_running before every read and write, so after a drop:

• the log says why, e.g. Telemetrix link lost (OSError(6, 'Device not configured')) - marking as disconnected
• the board goes DISCONNECTED, and the next write raises Board not connected
• MainWindow._on_hardware_connection_change pauses the running experiment and shows the hardware-disconnection dialog (existing behavior)

Not covered

An Arduino that resets without the USB link dropping (motor noise, brownout) keeps the port open, so writes still go through and the reset board ignores them. Catching that needs a heartbeat to the board.

Testing

• New test_dropped_serial_link_reads_as_disconnected: fails on main, passes here.
• Checked against telemetrix-aio's real _arduino_report_dispatcher and TelemetrixAioSerial with a port that drops partway through a run: the board reports disconnected while the thread is still alive, and the next write raises.
• tests/: 4759 passed. Plugin suites were not run locally.
• ruff and black are clean on the changed files.
• Not yet tried on a real Arduino.