Draw civil flights from an icon set moved on the GPU

Every flight is now a row in the layer's icon set. Its motion segment
follows the same regime `_deadReckon` picks (interpolate between fixes,
warm up backward from the oldest, coast forward from the newest), and
one shared `_motionRegime` makes that choice for both. The shader moves
every untracked contact between polls, so the 12 Hz fleet tick no
longer touches each billboard. It now only re-plans expired rows,
floors grounded contacts, advances focus while needed, and runs the 3D
model regime.

Grounded contacts the floor lifts are pinned to a hold at the floored
position. Their nose glides on the GPU. Contacts within 50 km re-floor
every tick, and farther ones take turns about once a second. Low
contacts are judged for the horizon at no lower than 12 m above the
ellipsoid, as before. Below hover speed a contact's nose holds. A row
whose fleet model is drawn keeps its focus updated, because the model
reads it. Untracking re-plans and re-floors the icon from the tracked
course.

On the same views with about 6,700 OpenSky flights, frame gaps at p95
drop from 28-40 ms to 17 ms, and the fleet tick from 16-37 ms to about
2 ms at most.

src/data/aircraftIcons.js

@@ -252,6 +252,8 @@ const _b64 = (s) =>
  *  NEAR the display size lets the browser's SVG AA do the work: crisp AND
  *  smooth. 64 covers the 40–58 px fleet band with ≤1.6× minification. */
 const FLEET_RASTER_PX = 64;
+/** Raster size of the default fleet glyph. */
+export const FLEET_ICON_PX = FLEET_RASTER_PX;
 /** Tracked raster: the tracked billboard is the one SUSTAINED large 2D glyph
  *  (close-zoom fleet flybys hand off to 3D models). Keep the 192 px texture so
  *  it stays crisp at its biggest on-screen sizes. */