perf(cables): show landing markers a few per frame

Switching Submarine cables on after the globe had loaded held the page
for about 1.4 s in one task. 1.2 s of it was Cesium reading the terrain
height for each of the 1,917 ground-clamped landing markers as it created
them, all in the first frame, including the ones behind the globe that
the horizon sweep hid a moment later. The markers now start hidden and
the sweep shows the visible ones 48 per rendered frame.

Co-Authored-By: Claude Fable 5.2 <[email protected]>

docs/CURRENT-STATE.md

@@ -8158,6 +8158,12 @@ Cesium's multipart suffixes. Each segment gets its own feature's styling, click
 destination and horizon-culling reference, and each source feature has one
 reference stem. The source collections are not modified.
 
+Landing markers are ground-clamped billboards, and Cesium reads the terrain
+height as it creates each one. They start hidden and the horizon sweep shows
+the visible ones 48 per rendered frame (`CABLE_LANDING_REVEALS_PER_FRAME`),
+so switching the layer on does not do every read in one task; hiding is still
+immediate.
+
 Natural Earth regions and neighborhood polygon lookup are package exports.
 Their existing lazy loaders, retry behavior, bundled datasets and attribution
 are unchanged. The cable dataset remains CC BY-NC-SA 3.0 and is not covered by