The timeline marks launches and strong earthquakes, not every weather frame

The timeline drew a tick for every registered mark, and the weather clock
registered every radar, satellite and lightning frame, which come every few
minutes, so the track was a wall of dashes and the arrow keys crept from
frame to frame.

Weather products now register only the span of their frames (start and
end, as before) with no marks. Holds, the gate and the next-frame wait
while the world plays are unchanged. A weather row's step param goes to
the previous or next weather frame (every shown product's frames, and the
row's own while it is suspended) rather than to the timeline's marks.

The Earthquakes layer registers with the world clock once, at init, and
unregisters on destroy. While on, its quakes of magnitude 6 and up
(EARTHQUAKE_MARK_MAGNITUDE) are marks, and its coverage is their own span,
or none without any, so it never stretches the timeline beyond the feed.
Off, it has none. It tells the clock when the set of marks changes. The
map still draws every M2.5+ quake.

Launches stay marks. With no mark ahead or behind, the arrow keys step
along the track as shown.

docs/CURRENT-STATE.md

@@ -1781,7 +1781,11 @@ Earthquake rendering is exposed through `./layers/earthquakes`. The layer owns
 its entities and request lifecycle; the application supplies the overlay host
 and snapshot source. The standalone adapter keeps the existing USGS daily feed,
 M2.5+ filtering, static discs, magnitude labels and analyst records. Disabling or
-destroying the layer cancels pending work and ignores late results.
+destroying the layer cancels pending work and ignores late results. While on,
+the layer registers its quakes of magnitude 6 and up
+(`EARTHQUAKE_MARK_MAGNITUDE`) with the world clock as timeline marks, spanning
+only their own times, and none while off; the map still draws every M2.5+
+quake.
 
 ## Rendered-mesh floor readings — November 9, 2026
 
@@ -8533,9 +8537,13 @@ Weather combines the GFS/ECMWF surface forecast with three observation layers:
 NOAA Rain radar (CONUS), Satellite clouds (regional GOES or slower global infrared),
 and 15-minute lightning density (Americas and Pacific). One transient observed
 clock moves all enabled observation rows with the world clock; the world
-timeline is the only time control. Each product's advertised times are marks on
-that timeline, which its arrow keys step to; each product selects its nearest
-frame at or before the world time, never a future frame or an interpolation.
+timeline is the only time control. Each product's advertised times span time on
+that timeline but are not marks on it: the timeline marks only events (launches,
+and earthquakes of magnitude 6 and up while that layer is on), so its arrow keys
+step to those, or along the track where there are none. A weather row's `step`
+param goes to the previous or next weather frame. Each product selects its
+nearest frame at or before the world 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, but never less than one and a half of the
 feed's median frame spacing over its latest frames, so a regional feed whose