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.

scripts/format-scope.json

@@ -1077,6 +1077,7 @@
   "src/layers/traffic/roadModes.test.mjs",
   "scripts/qa-traffic-motion.mjs",
   "src/layers/traffic/motion.test.mjs",
+  "src/layers/traffic/lineRebuild.test.mjs",
   "scripts/qa-journey-recorder.mjs",
   "scripts/qa-alpr-journey.mjs",
   "scripts/qa-context-arrivals.mjs",