SAFETY-90: the peak guard goes on after a P1 meter gap

A meter gap past meter_max_age_s left seconds the quarter average never
counted, so the guard was blind for the rest of the quarter (rig case 7b:
3964.8 W against a 3450 W aim). Once readings are back, the missed seconds
now count at the higher of the limit and the import seen just before and
just after the gap with the batteries at 0 W (peakguard.gap_w/fill_gaps);
the gap's first meter_max_age_s is topped up to that too. Seconds before
the add-on's first reading count at the aim, and the page says part of the
period was not measured (EN/NL). The bill record is unchanged; no option.

DECISIONS/SAFETY-90-peak-guard-after-a-gap.md, test_safety90.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

goodwe_controller/app/ui_text/en.json

@@ -410,6 +410,7 @@
     "step_in": "The batteries step in above {kw}.",
     "step_in_would": "The batteries would step in above {kw}.",
     "period": "This 15-minute period is heading for {kw}.",
+    "period_unmeasured": "Part of this 15-minute period was not measured.",
     "prices": {
       "now": "Price now {eur} per kWh, all in.",
       "today": "Today: lowest {lo} at {lo_at}, highest {hi} at {hi_at}.",