TARIFF-24 round 2: a usable solar forecast replaces the "no weather" x1.5 (AUDIT-02 F1)

With battery modes on, the price gap a mode needs was always widened x1.5 by
Uncertainty.NO_WEATHER, even with a solar forecast read, so a 0.08 day/night
gap never traded at the shipped cycle cost. The doubt is now added only when
no usable forecast is read (solar_fc is None, the day plan's own test), in the
run plan (_price_runs), the charge gate (_mode_roles) and Home's mirror
(homeview.trade_verdict). The rule is one helper, arbitrage.weather_uncertainty();
the modes-off trade path (main.py cycle) keeps its literal NO_WEATHER.

DOCS "Battery modes" sentence, CHANGELOG 0.19.1 line and the DECISIONS note
follow. Tests: test_tariff24_weather.py.

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

goodwe_controller/DOCS.md

@@ -538,7 +538,7 @@ With battery modes on, these choices replace the *Charging and selling on price*
 
 **Cheap and expensive hours.** They come from the published day-ahead prices, as runs of consecutive 15-minute periods, not a single period.
 - The cheap run is the cheapest stretch long enough to fill the batteries that may charge, at their power.
-- An expensive hour costs at least *Battery cycle cost* (`cycle_cost_threshold_eur`, widened as for trading) more than the cheap run. The batteries hold their power until then.
+- An expensive hour costs more than the cheap run by at least *Battery cycle cost* (`cycle_cost_threshold_eur`) when a solar forecast is read, and by 1.5 times that when none is (a real-time price feed widens it further). The batteries hold their power until then.
 - The sale uses the most expensive stretch inside those hours, and never a period below 0.
 - A carried (not yet published) price is never used. A run that has started runs to its end.
 - With no prices, or too small a gap, every battery follows the house.