perf(traffic): build road lines one set at a time

With Lines on, every progressive paint of a loading city rebuilt all road
lines: up to three ground-line primitives for as many as 1,500 roads, built
asynchronously. Builds started faster than they finished, and each kept the
set it replaced in the scene until it was ready, so sets piled up.

One set now builds at a time. A rebuild asked for while lines are building
replaces the one waiting, and the newest starts when the build finishes.
Lines replaced by a finished build are removed before the next one starts.
Turning Lines off removes the lines at once and drops the waiting rebuild.

docs/CURRENT-STATE.md

@@ -134,7 +134,7 @@ existing batch size and one bounded upstream chunk. Larger requests return HTTP
 413 before disk-cache loading or upstream work; malformed coordinates remain 400.
 The browser continues to split larger workloads into sequential 64-point requests.
 
-Traffic holds continuous rendering only while enabled with shown animated dots, a pulsing jam heat-line, or road lines whose geometry is still building (replaced lines stay drawn until their successors are ready, so a rebuild never blinks); hidden dots (`dots` off) and settled flow lines hold nothing. Empty and high-altitude views release the hold. A failed load leaves no hold only when no animated content is retained: if an earlier load succeeded, ingestion keeps its animated points and the hold stays. Population admission reacquires the hold and resets animation time. Clear, disable, and destroy also release it. Retained fading dots keep animation alive until they are removed. When the last point leaves mid height correction, the correction is settled at once, before the hold is released, so it cannot keep the layer reporting loading.
+Traffic holds continuous rendering only while enabled with shown animated dots, a pulsing jam heat-line, or road lines whose geometry is still building (replaced lines stay drawn until their successors are ready, so a rebuild never blinks; one set builds at a time, and of the rebuilds asked for meanwhile only the newest runs, when that build finishes; turning lines off removes them at once); hidden dots (`dots` off) and settled flow lines hold nothing. Empty and high-altitude views release the hold. A failed load leaves no hold only when no animated content is retained: if an earlier load succeeded, ingestion keeps its animated points and the hold stays. Population admission reacquires the hold and resets animation time. Clear, disable, and destroy also release it. Retained fading dots keep animation alive until they are removed. When the last point leaves mid height correction, the correction is settled at once, before the hold is released, so it cannot keep the layer reporting loading.
 
 Shared distance checks use `src/geoDistance.js` with `{ lat, lon }` points and
 the IUGG mean spherical Earth radius, 6371.0088 km. `greatCircleKm` is