SAFETY-118: read the peak forecast in its own unit

peak_forecast_entity is read through pv.watts() by its unit_of_measurement;
the abs(raw) < 50 size rule is gone. No unit or a non-power unit means the
forecast is not used, with one warning per cause. The validator no longer
claims a kW sensor is taken as watts for this entity.

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

goodwe_controller/app/pv.py

@@ -93,12 +93,12 @@ def watts(state: dict | None) -> tuple:
     unknown" are different claims and the page has to be able to make the first.
 
     ⚠️ THE UNIT IS READ OFF THE ENTITY AND NOT ASSUMED, AND THERE IS NO
-    MAGNITUDE HEURISTIC. `read_inputs()` guesses kW-or-W for
-    `peak_forecast_entity` with `abs(raw) < 50`, which is safe there because
-    nobody's quarter-hour demand forecast is 50 W. It is NOT safe here: 30 W of
-    real production at dusk is an ordinary reading, and that heuristic would
-    publish it as 30 kW - a fabricated number, in the channel whose entire
-    purpose is to be believed a week later. A sensor whose unit this cannot read
+    MAGNITUDE HEURISTIC. 30 W of real production at dusk is an ordinary reading,
+    and a "small number means kW" rule would publish it as 30 kW - a fabricated
+    number, in the channel whose entire purpose is to be believed a week later.
+    SAFETY-118 removed the last such rule, on `peak_forecast_entity`, for the
+    same reason (DECISIONS/SAFETY-118-peak-forecast-unit.md): the peak forecast
+    now reads through this function too. A sensor whose unit this cannot read
     is REFUSED rather than guessed at.
     """
     if state is None:
@@ -126,6 +126,24 @@ def watts(state: dict | None) -> tuple:
     return value * WATTS_PER[unit], unit
 
 
+def unit_fault(state: dict | None):
+    """SAFETY-118. `None` unless `state` holds a number whose unit cannot be
+    read in watts, then that unit (`""` for none). A missing, unavailable or
+    non-numeric state is a missing reading, not a unit fault, so it is `None`.
+    """
+    if state is None:
+        return None
+    raw = str(state.get("state", "")).strip()
+    if raw.lower() in BAD_STATES:
+        return None
+    try:
+        float(raw)
+    except ValueError:
+        return None
+    unit = str((state.get("attributes") or {}).get(UNIT_KEY) or "").strip()
+    return None if unit in WATTS_PER else unit
+
+
 # --- TEL-12: the ROSTER, and the SITE figure over it --------------------------
 
 def entities(opts: dict) -> tuple: