OPT-27: the night plan keeps the last fresh solar forecast

Forecast.Solar's sensors stop updating in the evening, so at night every such
site had no forecast: the night cheap run needed 1.5x the cycle cost and
Leftover solar planned nothing. One missed poll also dropped it at once.

solarfc.Hold keeps the last fresh reading of each sensor, for the day it is
for. A missing or stale sensor is covered by it for up to FRESH_S (a short
outage), and from the end of the bell (20:00) until local midnight + 3 h
however old. A tomorrow figure is tomorrow's until midnight and then today's;
a today figure is only today's. Stored in the controller's store
(solar_fc_hold), so an evening restart keeps it. The forecast line says
'Solar forecast read 17:05, used for tonight's plan.' (EN, NL).

test_tariff24_weather.py runs with a hold that never covers: it tests what the
plan does when the forecast IS lost.

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

goodwe_controller/DECISIONS/OPT-27-last-fresh-forecast-for-the-night.md

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+# OPT-27 - the night plan keeps the last fresh forecast
+
+**Status: done on `release/1.0`, not released.** Tests: `test_opt27.py`.
+
+**The defect.** A today sensor is stale after 3 h (`FRESH_S`) or when it is from
+before local midnight. Forecast.Solar stops updating in the evening, so at night
+every Forecast.Solar site had no forecast: a price mode's night cheap run needed
+1.5x the cycle cost (TARIFF-24 round 2) and Leftover solar planned nothing.
+Separately, one missed poll (`unavailable`) dropped the forecast at once.
+
+**Where: `solarfc`, not the plan.** The last fresh reading of each sensor is
+kept in `solarfc.Hold` and `read_sum()` uses it when a sensor has none. A hold
+at the plan (`_wx_track`/`weather_held`) can only keep the 1.5x gap off. The
+day plan, Leftover solar's levels, Home and the Forecast tab all read
+`Controller.solar_fc`, so one kept forecast makes control and every screen agree
+(UX-53) with no second rule to keep in step. `weather_held()` is unchanged: it
+still keeps a run under way when a read is lost.
+
+**The rule** (`Hold.stand_in()`). A kept reading is for one day. It covers a
+sensor that is missing or stale, never one with a bad value or unit, and:
+- for up to `FRESH_S` when the sensor is missing: a short outage, any time of day;
+- from the end of the bell (20:00 local, `BELL_TO_H`) until local midnight + 3 h
+  (`KEPT_UNTIL_H`), however old;
+- the day it is for: a today figure only on that day, a tomorrow figure as
+  tomorrow until midnight and, if the today sensors are still stale, as today's
+  until midnight + 3 h (all sensors of the sum are needed, as for any sum);
+- until the next fresh reading, which replaces it at once.
+
+So a figure is never used for another day and never older than one night.
+Daytime is as before: a sensor that stopped is stale after 3 h, and no forecast
+all day is still no forecast (the night run still needs 1.5x).
+
+**Where it is stored.** The controller's store, key `solar_fc_hold`, so an
+evening restart keeps it. It is written when a reading's figure changes or
+every 15 min (`SAVE_EVERY_S`), not on every poll. A damaged entry is dropped.
+
+**The screens.** `Forecast.kept` is true when a reading older than `FRESH_S` is
+in use; the forecast line then reads "Solar forecast read 17:05, used for
+tonight's plan." (`home_words.solar.kept`, EN and NL). The event log says
+"read, kept from 17:05".
+
+**Not changed.** Modes off (nothing reads the sensors). A forecast that stays
+fresh. The hold does not look at the weather, only at the clock and the day.