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/DECISIONS/SAFETY-118-peak-forecast-unit.md

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+# SAFETY-118 - the peak forecast is read in its own unit
+
+**Status: for review.** Tests: `test_safety118.py`.
+
+`peak_forecast_entity` was read as a bare number, and anything under 50 was
+multiplied by 1000 ("accept kW or W"). A W sensor reading 17 to 45 W, which a
+real site did 430 times in one morning, was taken as 17 to 45 kW. The validator
+also called a kW sensor wrong, which was false for this entity.
+
+Decision: the size rule is gone. The entity is read through `pv.watts()`, the
+same unit table and reader as the solar entities, so W, kW, MW and the rest
+convert by the sensor's own `unit_of_measurement`. No unit, or a unit that is
+not power (kWh), means the forecast is not used, exactly as if the option were
+empty: the loop cap and every non-forecast peak step still apply. One WARNING
+per cause, and a status row and a settings note say the same.
+
+No size rule can be right in both units: 30 W and 30 kW are both ordinary.
+The read is still one `GET /api/states/<id>` per cycle, as `hass.number()` made.