startNavigation
Starts turn-by-turn navigation with the currently selected route.
Called when the user taps "Navigate" on Android Auto route preview. This triggers:
GuidanceViewModel to start guidance
isGuidanceActive to become true
guidanceState to start emitting navigation updates
Preconditions
A route must be computed and selected
Route preview should be active
Implementation Notes
The adapter should call guidanceViewModel.onEvent(GuidanceEvent.OnStartGuidance).