fix(transit): keep terrain heights per point so rereads stop freezing the page

On a terrain globe every floor reread asked globe.getHeight, a ray cast against
the drawn tile's mesh, for every trail fix of every vehicle in view, twice per
pass and seven passes per poll. Over Boston's 650 vehicles that was about
150,000 reads per 10 s for about 8,000 distinct points, and blocked the page
for up to 19 s at a time while zooming.

Heights are now kept per point (1e-5 degrees), read again only while the camera
moves or tiles load, and read within a time budget: vehicle positions first,
the path a marker is drawn along next, undrawn trail history last.

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

changelog.d/0000-transit-terrain-height-cache.md

@@ -0,0 +1,6 @@
+- Fix the page freezing for seconds at a time with Transit on over a terrain
+  globe, most visibly while zooming over a large fleet. Every reread asked the
+  terrain for the height of every trail fix of every vehicle in view; heights
+  are now kept per point, read again only while the camera moves or tiles
+  load, and read within a time budget that puts vehicle positions first
+  (mazy1998).