OPT-36: Leftover solar counts what Cheap hours and Solar first batteries will give before buying

Live 0.20.0: A (Leftover solar, 60 %) charged to 95 % while B (Cheap hours,
94 %) already covered the expensive hours. dayplan.plan() now takes off the
shortfall what Cheap hours batteries will hold when the hours start (the
ceiling or what their cheap run reaches under the peak aim) and what Solar
first batteries will still hold after the night's use. Home reads a give
block after the planned charge, plan_cover() is the plan's own figure, and
the Why line says other batteries cover.

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

goodwe_controller/DOCS.md

@@ -549,7 +549,7 @@ With battery modes on, these choices replace the *Charging and selling on price*
 - The hourly shape is kept only if every sensor has one. Otherwise the total is spread over the day as for Forecast.Solar.
 - Before each cheap run the add-on asks: will the sun fill the batteries before the expensive hours? It counts on `solar_trust_pct` of the forecast (70 %), less what the house uses. Solar first batteries get that sun first.
 - *Leftover solar* then charges from the grid only the shortfall: for each expensive hour, what the house uses less the sun counted for that hour (never below 0), added up over all the expensive hours, early morning and evening. With a day and night tariff that is 07:00 to 22:00. Not up to the top level.
-- All *Leftover solar* batteries hold that shortfall together, shared by their size. A battery that reaches the top level passes the rest to the others. If all of them together cannot hold it, they all fill to the top level. In the expensive hours the rest comes from another battery (*Cheap hours only* or *Solar first*) or from the grid. The Batteries screen says so. The sun before the expensive hours still fills first, and is taken off what is bought.
+- All *Leftover solar* batteries hold that shortfall together, shared by their size. A battery that reaches the top level passes the rest to the others. If all of them together cannot hold it, they all fill to the top level. *Cheap hours only* and *Solar first* batteries give first: what they will hold when the expensive hours start (a *Cheap hours only* battery full, or as far as its cheap run reaches under the peak limit; a *Solar first* battery after the night's use) is taken off the shortfall. In the expensive hours the rest comes from the grid. The Batteries screen says so. The sun before the expensive hours still fills first, and is taken off what is bought.
 - A forecast that gives only one figure for the day (Forecast.Solar) is spread over the hours from now, so the hours add up to that figure.
 - *Cheap hours only* ignores the forecast and fills to the top level.
 - *Solar first* never charges from the grid.