Fix scrubs to live on long tracks, fire ages in a past window, and gaps between hourly cloud frames

- World timeline: a scrub counts as at now within a share of the track
  (0.5%), now held to at most two minutes. A layer whose source covers
  years made the window weeks wide, so most of the last month snapped to
  live. Short tracks keep their window.
- Fires: card and detail ages count back from the end of the 24 hours the
  layer shows when its snapshot ends at a set time, and from now only when
  it runs to now. A window that ended months ago read "new 105d" on fires
  three hours old at the time shown.
- Weather: a product's max gap and the delay after which its latest frame
  reads late are at least one and a half of its feed's median frame
  spacing (over its latest frames). A regional cloud feed with frames an
  hour apart drew nothing for the second half of every hour while
  scrubbing and read late after twenty minutes; feeds with frames minutes
  apart keep their gaps.

docs/CURRENT-STATE.md

@@ -3455,7 +3455,8 @@ reads `NASA FIRMS · LIVE · updated …`. A source may instead answer the 24 ho
 to a set time, naming its end as `at` (epoch ms): the row then reads
 `NASA FIRMS · 24 h to MM-DD HH:MM UTC`, `getStats()` reports `at`, and
 `get_active_fires` says "in the 24 hours to" that time and reports it as
-`window_end`. The dev proxy never sets `at`.
+`window_end`, and card and detail ages count back from that time instead of
+now. The dev proxy never sets `at`.
 
 The FIRMS CSV parser rejects impossible acquisition calendar dates before the proxy filters records to the trailing 24 hours, while retaining valid UTC acquisition times.
 FIRMS ages round to whole hours before choosing a unit, so a 47.6-hour age reads "2d", not "48h"; with detections filtered to 24 hours, this shows mostly in the `STALE · cached {age}` stats line.
@@ -8411,7 +8412,11 @@ clock drives all enabled observation rows. Earlier/Later use the sorted union of
 non-suspended products' advertised times; each product selects its nearest frame
 at or before the requested UTC time, never a future frame or an interpolation.
 The maximum gap is 30 minutes for radar, regional infrared and lightning, and
-3 hours for hourly global infrared. A missing eligible frame hides that product
+3 hours for hourly global infrared, but never less than one and a half of the
+feed's median frame spacing over its latest frames, so a regional feed whose
+frames come an hour apart holds each for 90 minutes; the latest frame reads late
+after the same floor (20 minutes for radar and regional infrared, 45 for
+lightning, 4 hours for global infrared). A missing eligible frame hides that product
 and reports the gap; rows show actual displayed UTC time with synced/nearest labels.
 Manifest rollover preserves the requested history time, even when frames expire.