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/data/layerState.js

@@ -335,6 +335,9 @@ const OPTION_GROUPS = Object.freeze({
       osm: 'o',
       hybrid: 'h',
     }),
+    // Street Traffic row toggles: flow-colored road lines, moving dots.
+    booleanOption('lines', 'l', false),
+    booleanOption('dots', 'd', true),
   ]),
   'weather-lightning': Object.freeze([
     enumOption('opacity', 'o', 'strong', ['light', 'strong'], {