feat: DependencyState.onViewModel for VM-backed dependencies (v2.18.0)

[JhonaCodes]

What

Adds change.onViewModel<VM, T>() to DependencyState, so a ViewModel can declare a dependency on a ViewModel-backed service (ReactiveNotifierViewModel<VM, T>) directly — instead of passing the raw ReactiveNotifier container as change.on<T>() requires today.

@override
void onDependenciesStateChanged(DependencyState change) {
  change.onViewModel<ProfileViewModel, ProfileModel>(
    ProfileService.state,
    (previous, current) {
      if (previous.id != current.id) _refreshFor(current.id);
    },
  );
}

Why

change.on<T>() only accepts a raw ReactiveNotifier. Passing a VM-backed service's container did not react to the inner ViewModel's updateState — a ReactiveNotifier<VM> container does not notify when the inner VM mutates. This makes cross-VM dependencies work ergonomically and correctly.

Changes

• DependencyState.onViewModel<VM, T>() — delegates to on<T>(service.reactiveNotifier, cb) with typed (previous, current).
• ReactiveNotifierViewModel.reactiveNotifier getter — exposes the underlying container.
• Dependency subscriptions (ViewModel and AsyncViewModelImpl) now listen to the inner ViewModel (ChangeNotifier) target, so reactions fire on updateState. Simple-type notifiers behave exactly as before.
• README section + CHANGELOG; version bumped to 2.18.0 (additive, minor).

Tests

New group in test/dependency_state_test.dart:

• setup snapshots the inner ViewModel data via a ReactiveNotifierViewModel
• reacts when the inner ViewModel mutates via updateState
• onViewModel equivalent to on(service.reactiveNotifier)

All 21 dependency tests pass. Full suite: only pre-existing golden env-mismatch failures remain (verified identical on clean main); no non-golden regressions.

Scope

Deliberately isolated to onViewModel. Does not include any of the ReactiveInspector WIP.