OPT-38 round 2: a kept tomorrow reading is flagged kept only from 20:00

Before the bell ends it replaced today's Home line and the event log with
the kept-for-tonight wording. stand_in() docstring covers the tomorrow case.

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

goodwe_controller/app/solarfc.py

@@ -332,8 +332,9 @@ class Hold:
 
         The kept reading must be for `day0`, and either (a) at most `FRESH_S`
         old, or (b) used at night: from the end of the bell to local midnight,
-        at most `KEPT_MAX_AGE_S` old. `kept` is true when (b) was needed,
-        or `as_today` (OPT-34: a tomorrow figure standing in for today, after
+        at most `KEPT_MAX_AGE_S` old; a "tomorrow" reading at any hour
+        (OPT-38; `read_sum()` flags it kept only from the end of the bell).
+        `kept` is true when (b) was needed, or `as_today` (OPT-34: a tomorrow figure standing in for today, after
         midnight and until the bell starts, `KEPT_MORNING_MAX_AGE_S`)."""
         rec = self._r.get((kind, entity, day0))
         if rec is None:
@@ -558,6 +559,9 @@ def read_sum(today, tomorrow, now: datetime, hold: Hold | None = None):
     if why0 is not None:
         return None, why0, gaps
     tday = _sum(tdays) if tomorrow and len(tdays) == len(tomorrow) else None
-    kept = kept or (tday is not None and tkept)
+    # OPT-38: before the bell ends a kept tomorrow reading is no "kept for
+    # tonight": it must not replace today's line or the log.
+    kept = kept or (tday is not None and tkept
+                    and _local_hour(now) >= BELL_TO_H)
     # The oldest member's report is when the sum was last fully known.
     return Forecast(_sum(days), tday, min(ats), kept), None, gaps