Phase 5: shared worker base, self-rescheduling polls, driver robustness

gui/qt_workers.py — one QueueWorker base replaces the per-device command
queue + dispatch + signal boilerplate. The loop blocks on the queue
instead of waking 10-20x/second forever (test_idle_worker_does_not_spin
asserts an idle worker burns ~no CPU). PollingQueueWorker adds
self-rescheduling polling: the next poll is queued only after the
previous finishes, so a device slower than the interval can't accumulate
a backlog (test_polling_never_overlaps_or_backs_up).

Helios responsiveness — the concrete bug that motivated the above: a free
running 1 s QTimer queued a status poll that took ~2 s, so the queue grew
for as long as the panel stayed connected.
- helios_laser._query reads until the CR terminator instead of sleeping a
  fixed 0.05 + 0.2 s per query
- one _query_int() helper replaces five copies of parse-with-logging
- polling is now driven by the worker; HeliosWindow's QTimer is gone
- dropped __del__, which disabled the laser and wrote to the serial port
  from the garbage collector at an unpredictable time

helios_test_app.py — the worker was moveToThread'd but every call site
invoked its methods directly, so all serial I/O (including the sleeps)
ran on the GUI thread; Query All froze the UI for ~2 s. Calls now go
through a queued signal to a pyqtSlot. Also: connect/disconnect cycles
leaked a QThread + worker + 9 connections each time; 16 copies of the
not-connected guard collapse to _require_connection(); the Query Power
button called a method that has never existed (AttributeError popup) and
is now disabled and documented in KNOWN_ISSUES.

DCBiasImageWidget preallocates its image and uses set_data/set_clim, so
the live preview stops rebuilding the array and the whole artist tree per
row (O(rows^2) over a scan).

bbd20x: connect() now raises when no bays respond instead of reporting
success on the wrong port; disconnect() joins with a timeout so a wedged
reader can't hang shutdown; one _channel_for() helper replaces four
copy-pasted axis mappings; hardcoded travel limits become TRAVEL_MM; the
joke error strings are gone.

gui/widgets.py adds the shared ConnectionBar / PortSelector / bounded
LogConsole / StatusGrid for the test benches to adopt. 65 tests passing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

KNOWN_ISSUES.md

@@ -17,6 +17,20 @@ If they appear, the setters need a stop-live/apply/restart sequence
 (re-create the helper around the two call sites in
 [uc480_camera.py](hardware/uc480_camera.py)).
 
+## Helios: no output-power query
+
+`docs/hardware/HELIOS_DRIVER_README.md` documents `driver.get_power_mw()`,
+but `HeliosLaser` has no such method and no output-power mnemonic appears
+anywhere in this repo's protocol notes. `helios_test_app.py` called it
+anyway and raised `AttributeError` into a popup; the button is now disabled
+and the handler reports the gap instead.
+
+**Bench check:** find the power-read command in the Helios manual (the
+other reads are three-letter mnemonics like `LDO`, `LDS`, `LTA`). If one
+exists, add `get_power_mw()` to `hardware/helios_laser.py` using
+`_query_int`, then re-enable the button. If it doesn't, delete the Power
+Monitoring group from the test app and fix the README.
+
 ## Genesis laser: forked protocol implementations disagree
 
 `hardware/genesis_core.py` and the reference implementation