This PR introduces an overload for sharedKoinViewModel to allow an optional navGraphRoute parameter.

Motivation
The current implementation relies on destination.parent?.route which works for simple hierarchies. In more complex navigation setups (nested graphs, sibling graphs, multi-level NavHost), developers may need explicit control over which NavBackStackEntry to use.

Changes
• Added navGraphRoute parameter with default this.destination.parent?.route.
• Fallback to default koinViewModel() if navGraphRoute is null.

Example Usage

val vm: MyViewModel = navBackStackEntry.sharedKoinViewModel(
    navController = navController,
    navGraphRoute = "route_graph"
)

This improves flexibility without breaking existing API.