EV-03 review: one power-unit table for the probe and the reading

POWER_UNITS is now derived from _W_PER, so the start-up probe and the
reading cannot drift apart. No behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

goodwe_controller/app/chargers.py

@@ -87,7 +87,9 @@ REQUIRED = ("current", "readback", "state", "power")
 _SETPOINT_RE = re.compile(r"^number\.(?P<x>.+)_" + SETPOINT_SUFFIX + "$")
 
 _ROLE_ORDER = tuple(ROLES)
-POWER_UNITS = ("mW", "W", "kW", "MW")   # every HA power unit, converted
+#: power unit -> W. The probe and the reading both use this one table.
+_W_PER = {"mW": 0.001, "W": 1.0, "kW": 1000.0, "MW": 1000000.0}
+POWER_UNITS = tuple(_W_PER)
 
 
 def census_key(entity) -> str:
@@ -155,9 +157,6 @@ def _amps(obj):
     return None
 
 
-_W_PER = {"mW": 0.001, "W": 1.0, "kW": 1000.0, "MW": 1000000.0}
-
-
 def _watts(obj):
     """Power in W, believed only in a power unit (its own copy of the rule: the
     PV channel's reader stays out of the charger's, test_pv.py part 6)."""