feat(transit): smooth coverage heat-map at every zoom, live feeds only

Draw coverage as Gaussian discs blended in the opaque pass: wide spots
from orbit cross-fading into service-area patches at regional zoom.
Translucent billboards darkened into rimmed discs under Cesium's OIT at
orbital distances. Feeds not yet seen live get no spot.

changelog.d/1716-transit-french-networks.md

@@ -18,8 +18,9 @@
 - Add 55 Japanese bus networks to the Transit layer, the first in Toyama,
   Mie, Kumamoto, Kōchi, Aichi, Okinawa and Aomori prefectures, each recorded
   under CC0 1.0 or CC BY 4.0 with the operator credited in Japanese (#1716).
-- Show where Transit has coverage from globe view. Above about 3,000 km,
-  where the layer draws no vehicles, each feed is a soft green spot and
-  regions with many networks glow hotter; the spots fade out as the camera
-  comes down to the height where vehicles appear. Drawn from the feed
-  registry, so no feed is polled for it (#1716).
+- Show where Transit has coverage. From orbit each feed is a soft green
+  spot, hotter where networks cluster; at continental and regional zoom the
+  spots become patches the size of each network's service area, drawn under
+  the vehicles; close in they are off. Drawn from the feed registry, so no
+  feed is polled for it, and only feeds whose live positions have been seen
+  are shown (#1716).