Traffic layer and traffic-aware ETAs

Apple Maps colours roads by live traffic, shows incidents, and gives traffic-aware ETAs. ts-maps has no traffic data layer, and turn-by-turn ETAs come only from the directions provider.

Scope

• A TrafficLayer that draws congestion along roads from a provider (vector tiles with a congestion property, e.g. Mapbox Traffic, or TomTom/HERE flow tiles), coloured green/yellow/red/dark red as Apple does.
• Incident markers (accidents, closures, roadworks) with a small card.
• TurnByTurn shows "traffic-aware" ETA and delay where the directions provider supplies it (Mapbox driving-traffic, Valhalla with live traffic, Google).
• A toggle in the map type picker (#6248).
• Bindings parity.