Add native iOS routing via MapKit MKDirections

[jkasprzyk17]

Context

MapViewDirections (#7) uses the Google Directions API, which requires an API key and billing. On iOS, MapKit provides native routing via MKDirections with no Google dependency — useful for apps that default to Apple Maps (provider="apple" after #1) or want to avoid Directions API costs on iOS.

react-native-maps-directions is Google-only. This issue adds an optional native routing path as a differentiator for @nitro-maps/directions.

Goal

Allow MapViewDirections to compute routes using MapKit on iOS when configured:

<MapViewDirections
  origin={origin}
  destination={destination}
  routingProvider="mapkit"  // or auto-detect from MapView provider
  mode="DRIVING"
  onReady={({ coordinates, distance, duration }) => { … }}
/>

Scope

• Define routing provider strategy:
  • routingProvider?: 'google' | 'mapkit' | 'auto'
  • auto: use MapKit on iOS when MapView uses Apple provider; Google elsewhere
• Implement iOS native module or Nitro hybrid (evaluate lightest approach):
  • wrap MKDirections / MKDirections.Request
  • support transport types mapping to mode where possible (automobile, walking, transit)
  • return decoded route polyline as Coordinate[]
  • return distance (meters → km) and expected travel time (seconds → minutes) for onReady parity
• Wire native adapter into @nitro-maps/directions fetch layer behind shared interface.
• Handle errors: no route found, network unavailable, unsupported transport type.
• Document platform limitations:
  • MapKit only on iOS
  • transit details may differ from Google
  • waypoints support per MKDirections capabilities
• Add example app toggle: Google Directions vs MapKit (iOS simulator).

Out of scope

• Android native routing (Google Directions remains default on Android).
• OSRM / GraphHopper for OpenStreetMap provider (Add OpenStreetMap provider on iOS and Android #3).
• Turn-by-turn navigation UI.
• Offline routing.

Acceptance criteria

• routingProvider="mapkit" renders a route on iOS without a Google API key.
• onReady returns coordinates, distance (km), and duration (minutes) consistent with Add MapViewDirections core — Google Directions API and route polyline #7 callback shape.
• Unsupported mode on MapKit fails gracefully with onError.
• Google routing (Add MapViewDirections core — Google Directions API and route polyline #7) still works unchanged when routingProvider="google".
• Docs explain when to use MapKit vs Google Directions and cost implications.

Implementation notes

Prefer a thin Nitro spec if other native routing adapters are planned (#1 multi-provider). Otherwise a minimal MKDirectionsModule may suffice for v1.

Shared interface sketch:

interface RoutingAdapter {
  fetchRoute(request: RouteRequest): Promise<RouteResult>;
}

Notes

Depends on #6 and #7. Can ship after Google Directions parity (#7–#9). Optional for v1.0 — Google-only directions may ship first.