A weather layer without a shared clock keeps its own on the world clock

Built without a weather clock, the weather layer still ran a frame replay
of its own: params.play stepped its frames every two seconds and went round
from the last to the first, params.step and params.latest picked frames
locally, and the layer tracked whether it followed the newest frame. That
was a second way to move time.

The layer now builds a weather clock on the page's world clock when none
is passed, and destroys it with the layer, so every layer follows the world
clock and its params move the world clock as the timeline does. The local
replay, its timer, the follow-latest flag, the wrap to the first frame, the
timerActive diagnostic and the reduced-motion "manual history available"
message are removed.

docs/CURRENT-STATE.md

@@ -8557,7 +8557,9 @@ and the timeline time alone at a past time.
 The catalog exposes `weatherClock` and destroys it with its lifetime signal. It
 has no time controls of its own: `weatherClock.world` is the world clock it
 follows, where time moves. A weather row's `play`, `latest` and `step` params
-move that world clock as the timeline does.
+move that world clock as the timeline does. A weather layer built without a shared
+clock keeps one of its own on the page's world clock for its lifetime; it has
+no frame replay of its own either way.
 
 Source, opacity and coverage navigation live in the WEATHER panel's cards, not
 in the rows; the panel shows the frame time and has no time controls. The