feat(characters:presentation): UI-agnostic MVI ViewModel (REDI-87)

- CharacterListState (characters, isLoading, isLoadingNextPage, currentPage, endReached, error: UiText?),
  CharacterListAction (OnCharacterClick/OnRetry/OnLoadNextPage), CharacterListEvent (NavigateToDetail/ShowSnackbar).
- CharacterListViewModel: state via .update, one-time events via Channel, DataError -> UiText on failure,
  pagination persisted in SavedStateHandle (rebuilds list up to the saved page after process death).
- CharacterUi + Character.toCharacterUi().
- NO Compose/Views deps: verified no androidx.compose on the compile classpath. Stability via
  ImmutableList instead of @Stable (which would require compose-runtime) — the only compose-named
  transitive is kotlinx-immutable's annotations-only stub, not the Compose framework.

feature/characters/presentation/build.gradle.kts

@@ -17,4 +17,7 @@ dependencies {
     implementation(libs.androidx.lifecycle.viewmodel.ktx)
     implementation(libs.androidx.lifecycle.viewmodel.savedstate)
     implementation(libs.kotlinx.coroutines.android)
+    // Stable collection for state — makes the list Compose-stable WITHOUT a Compose dependency,
+    // so this module stays UI-agnostic (no @Stable annotation, which would require compose-runtime).
+    implementation(libs.kotlinx.collections.immutable)
 }