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]>

docs/CURRENT-STATE.md

@@ -5578,6 +5578,23 @@ Noir samples also exclude its deliberately vignetted outer field, using shader
 settings and screen position rather than observed brightness. Selected-trail
 diagnostics report every condition even when selection fails.
 
+On a terrain globe Transit reads each vehicle's and trail point's height from
+the loaded surface (`globe.getHeight`, a ray cast against the drawn tile's
+mesh) and keeps the answer per point, about a metre apart
+(`src/layers/transit/height.js`, constants in `policy.js`). While the camera
+moves or tiles load a kept height is read again after 2 s; a height read once
+both have been still for 5 s is kept until either changes. Reads are budgeted
+at 16 ms per 200 ms window, 12 ms of it for re-reads, longest-kept first. A
+vehicle's own position is always read; the path its marker is drawn along
+comes next; the undrawn trail history of unselected vehicles may use a quarter
+of each budget. Past the budget a kept point keeps its last height, a new
+point takes the last height read in its ~111 m floor cell or waits, and the
+reread cycle continues until none waits. Before this, every reread asked the
+terrain for every trail fix of every vehicle in view, which blocked the page
+for up to 19 s at a time over a 650-vehicle fleet while zooming. Verified
+against Boston's MBTA feed on a keyless terrain globe only; no steeper city
+has a keyless feed to check against.
+
 Transit brackets use the normal-composite overlay surface: a 1.25 px mode-colour
 stroke over 3.25 px dark backing. Other contact themes retain their existing
 behavior. The selected trail uses a 3 px core over 5 px backing, mode-coloured