Validate precipitation probabilities and ice wording

docs/internal/facts.md

@@ -26,6 +26,8 @@ uses half-open period overlap to select hourly, narrative, daily, and alert
 data; it also derives precipitation timing. Convective outlooks are retained
 only when their valid interval overlaps the report period, with discussions
 kept for represented outlook days. Both collections are sorted deterministically.
+It rejects collected hourly data with a precipitation probability outside the
+finite 0 through 100 percentage domain before constructing derived facts.
 
 Report identity controls the summary shape: