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>

tests/test_helios_laser.py

@@ -41,19 +41,33 @@ class FakePort:
     asked for, not by discarding what it happens to find.
     """
 
-    def __init__(self, replies=None, timeout=1.0):
-        self.replies = REPLIES if replies is None else replies
+    def __init__(self, replies=None, timeout=1.0, discard_writes=False):
+        self.replies = dict(REPLIES) if replies is None else replies
         self.timeout = timeout
         self.is_open = True
+        # "Commands or set values can be discarded by the controller
+        # unintentionally" (manual, Section 6) — the case a verified write
+        # exists to catch.
+        self.discard_writes = discard_writes
         self.written: list[str] = []
         self._buf = bytearray()
 
+    def _store(self, mnemonic: str, value: str):
+        """Keep a written value, so a later query reads back what was set."""
+        if self.discard_writes:
+            return
+        previous = self.replies.get(mnemonic, [f"{mnemonic} = 0"])[0]
+        unit = previous.split()[3:]     # "LDS = 1500 mA" -> ["mA"]
+        self.replies[mnemonic] = [" ".join([f"{mnemonic} =", value, *unit])]
+
     # ── the bits of pyserial.Serial the driver uses ──────────────────────────
 
     def write(self, data: bytes) -> int:
         text = data.decode("ascii").strip()
         self.written.append(text)
         fields = text.split()
+        if len(fields) > 1:
+            self._store(fields[0].upper(), fields[1])
         for line in self.replies.get(fields[0].upper() if fields else "", []):
             self._buf += line.encode("utf-8") + b"\r\n"
         return len(data)
@@ -96,6 +110,18 @@ def laser():
     drv.serial = FakePort()
     drv.is_connected = True
     drv.TRAILING_QUIET_S = 0.0
+    drv.SET_SETTLE_S = 0.0
+    return drv
+
+
+@pytest.fixture
+def stubborn_laser():
+    """A controller that answers every query but keeps its own set values."""
+    drv = HeliosLaser(timeout=1.0)
+    drv.serial = FakePort(discard_writes=True)
+    drv.is_connected = True
+    drv.TRAILING_QUIET_S = 0.0
+    drv.SET_SETTLE_S = 0.0
     return drv
 
 
@@ -158,3 +184,54 @@ def test_raw_command_returns_every_line(laser):
     assert laser.send_raw_command("LCE") == (
         "LCE = 2\nBit 15..0: 0000 0000 0000 0010"
     )
+
+
+# ── Set values the controller may discard ────────────────────────────────────
+#
+# Section 6 of the 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."  A write that
+# reports success without reading back leaves the panel showing a setpoint
+# the laser never took, until the next poll replaces it with the old value.
+
+def test_a_set_current_is_read_back(laser):
+    assert laser.set_current_ma(900) is True
+    assert laser.get_current_ma() == 900
+    assert "LDS 900" in laser.serial.written
+
+
+def test_a_discarded_set_current_is_retried_then_reported(stubborn_laser):
+    assert stubborn_laser.set_current_ma(900) is False
+    # Retried, not given up on after one write.
+    assert stubborn_laser.serial.written.count("LDS 900") == \
+        stubborn_laser.SET_RETRIES
+    # And the controller's own value is what it still holds.
+    assert stubborn_laser.get_current_ma() == 1500
+
+
+def test_a_set_that_takes_on_a_retry_succeeds(laser):
+    """One dropped write, then the controller accepts — still a success."""
+    real_write = laser.serial.write
+    state = {"drops": 1}
+
+    def flaky(data: bytes) -> int:
+        if data.decode("ascii").strip().startswith("LDS ") and state["drops"]:
+            state["drops"] -= 1
+            laser.serial.written.append(data.decode("ascii").strip())
+            return len(data)        # swallowed: nothing stored, no reply
+        return real_write(data)
+
+    laser.serial.write = flaky
+    assert laser.set_current_ma(900) is True
+    assert laser.get_current_ma() == 900
+
+
+def test_a_set_frequency_is_read_back(laser):
+    assert laser.set_frequency_hz(25000) is True    # 40000 ns
+    assert laser.get_frequency_hz() == 25000
+    assert "LDF 40000" in laser.serial.written
+
+
+def test_an_out_of_range_current_is_not_sent(laser):
+    assert laser.set_current_ma(9000) is False
+    assert laser.serial.written == []