feat(traffic): Lines and Dots toggles; hosts choose which road modes they offer

createTrafficSource({ roadModes }) lets a host offer a subset of the TomTom,
OSM and Hybrid road modes (all three by default). Without OSM the layer never
draws simulated roads, and a single mode shows no road chip.

While flow is live the Street Traffic row offers Lines (every road with flow
as a steady green/amber/red line, from the heat-line renderer) and Dots (the
moving dots), saved in layer state and share links. Defaults: Lines off,
Dots on.

src/layers/traffic/index.js

@@ -24,7 +24,7 @@ export function createTrafficLayer({ services, source }) {
     ].every((key) => typeof source?.[key] === 'function')
   )
     throw new TypeError('A traffic source is required');
-  const state = createState({ services });
+  const state = createState({ services, source });
   const parts = {};
   const context = { state, services, parts, source };
   parts.style = createStyle(context);