REDI-94: GetCharactersPageUseCase + inject into list ViewModel

Add a domain UseCase (operator invoke) in :feature:characters:domain delegating
to CharacterRepository, and have CharacterListViewModel depend on it instead of
the repository directly. The UseCase is a deliberate thin pass-through that
documents the 'when to add a UseCase' convention (real logic / multi-source
composition vs. a single forwarded call).

feature/characters/domain/src/main/kotlin/com/example/architecture/feature/characters/domain/usecase/GetCharactersPageUseCase.kt

@@ -0,0 +1,27 @@
+package com.example.architecture.feature.characters.domain.usecase
+
+import com.example.architecture.core.domain.DataError
+import com.example.architecture.core.domain.Result
+import com.example.architecture.feature.characters.domain.CharacterRepository
+import com.example.architecture.feature.characters.domain.model.CharactersPage
+
+/**
+ * Loads one page of characters.
+ *
+ * **When to add a UseCase (convention note):** introduce a UseCase when a screen needs business
+ * logic that does NOT belong in the ViewModel — non-trivial rules, or *composition* of several
+ * repositories/sources into one domain operation. When the ViewModel would merely forward a single
+ * repository call, skipping the UseCase and injecting the repository directly is perfectly fine.
+ *
+ * This particular UseCase is a **thin pass-through, included for illustration**: it adds no logic
+ * beyond delegating to [CharacterRepository]. It earns its place only as a showcase of the
+ * convention (domain-owned, `operator fun invoke`, constructor-injected). In a real app you would
+ * grow it the moment list loading gained real behaviour (filtering, merging a local cache, …) — or
+ * delete it and let the ViewModel call the repository.
+ */
+class GetCharactersPageUseCase(
+    private val characterRepository: CharacterRepository,
+) {
+    suspend operator fun invoke(page: Int): Result<CharactersPage, DataError> =
+        characterRepository.getCharacters(page)
+}

feature/characters/presentation/src/main/kotlin/com/example/architecture/feature/characters/presentation/CharacterListViewModel.kt

@@ -8,7 +8,7 @@ import com.example.architecture.core.domain.onFailure
 import com.example.architecture.core.domain.onSuccess
 import com.example.architecture.core.presentation.UiText
 import com.example.architecture.core.presentation.toUiText
-import com.example.architecture.feature.characters.domain.CharacterRepository
+import com.example.architecture.feature.characters.domain.usecase.GetCharactersPageUseCase
 import com.example.architecture.feature.characters.presentation.model.CharacterUi
 import com.example.architecture.feature.characters.presentation.model.toCharacterUi
 import kotlinx.collections.immutable.toImmutableList
@@ -25,7 +25,7 @@ import kotlinx.coroutines.launch
  * via a [Channel], maps failures to [UiText], and persists the loaded page in [SavedStateHandle].
  */
 class CharacterListViewModel(
-    private val characterRepository: CharacterRepository,
+    private val getCharactersPage: GetCharactersPageUseCase,
     private val savedStateHandle: SavedStateHandle,
 ) : ViewModel() {
 
@@ -62,7 +62,7 @@ class CharacterListViewModel(
 
             var page = 1
             while (page <= targetPage) {
-                when (val result = characterRepository.getCharacters(page)) {
+                when (val result = getCharactersPage(page)) {
                     is Result.Success -> {
                         accumulated += result.data.characters.map { it.toCharacterUi() }
                         lastLoadedPage = page
@@ -123,7 +123,7 @@ class CharacterListViewModel(
         // get appended twice.
         _state.update { it.copy(isLoadingNextPage = true, error = null) }
         viewModelScope.launch {
-            characterRepository.getCharacters(page)
+            getCharactersPage(page)
                 .onSuccess { pageData ->
                     _state.update { state ->
                         state.copy(