Helios diode current: stop the status poll overwriting the setpoint

The current spin box could not hold a typed value: the 1 Hz status poll
read LDS and wrote it straight into the spin box, so the operator's number
was replaced by the laser's within a second — before Set could be pressed.
The spin box is now seeded once on connect and belongs to the operator
after that; the poll's reading goes to a read-back label beside the Set
button, so the value about to be sent and the value the laser holds are
separate readouts.

The write itself was also unverified. Section 6 of the operator's manual:
"Commands or set values can be discarded by the controller unintentionally.
It is recommended to query the set value after the command is entered to
confirm the actual value." set_current_ma() wrote LDS and returned True
regardless, so a discarded write looked exactly like a good one.
_write_verified() now writes, reads back, and retries up to three times;
set_current_ma() and set_frequency_hz() use it, and HeliosWorker reports a
refusal on the status line instead of echoing the requested value.

Also from the manual, recorded but not acted on: LDS accepts 0-7000 mA
(the driver's 2000 mA ceiling is this rig's, not the protocol's), LDF has
to be re-sent after LDG changes, and the power-monitor mnemonic is HMP —
which this laser does not implement, per the operator.

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

KNOWN_ISSUES.md

@@ -25,11 +25,12 @@ 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.
+**Answered (2026-09-08):** the mnemonic is `HMP` (Table 6-3, "Laser Power
+Monitor", read-only, 0-5000 mW) — but the table adds "not available on all
+models", and the operator confirms this rig's laser has no power meter.  So
+the button stays disabled; what remains is to delete the Power Monitoring
+group from `helios_test_app.py` and drop the power-monitoring claims from
+`docs/hardware/HELIOS_DRIVER_README.md`.
 
 ## Genesis laser: forked protocol implementations disagree