test(fires): pin the 47.5-hour flip to days and name where the age shows

Co-Authored-By: Claude Opus 5.5 <[email protected]>

changelog.d/1531-firms-age-rounding.md

@@ -1,3 +1,5 @@
-- FIRMS fire ages round to whole hours before choosing a unit, so a detection
-  47.5–48 hours old reads "2d", as one exactly 48 hours old does, not "48h"
-  (@quanticsoul4772, #1531).
+- FIRMS fire ages round to whole hours before choosing a unit, so an age of
+  47.5–48 hours reads "2d", as exactly 48 hours does, not "48h". Detections
+  are filtered to the trailing 24 hours, so this shows mostly in the
+  `STALE · cached {age}` stats line after about two days without a successful
+  refresh (@quanticsoul4772, #1531).