feat(characters:data): DTOs, mappers, data source, repo, Koin module (REDI-86)

- @Serializable CharacterDto/CharactersResponseDto/PageInfoDto in dto/.
- mappers/CharacterMapper.kt: internal, pure toDomain()/toCharacter()/toCharacterDetails();
  nextPage parsed from info.next URL. No mapping inside DTO/data-source classes.
- KtorCharacterDataSource via the typed HttpClient.get helpers (errors -> DataError.Network).
- NetworkCharacterRepository (not *Impl) maps DTO -> domain; DataError.Network widens to DataError.
- charactersDataModule: singleOf(::KtorCharacterDataSource) + singleOf(::NetworkCharacterRepository) { bind<CharacterRepository>() }.
- core:data: expose ktor-client-core as api (public inline helpers are inlined into consumers) and
  move Timber logging into a @PublishedApi internal fn so Timber doesn't leak across modules.

core/data/build.gradle.kts

@@ -19,4 +19,7 @@ android {
 dependencies {
     implementation(project(":core:domain"))
     implementation(libs.timber)
+    // `api`: the public inline HttpClient.get/post/delete helpers are inlined into consumer modules,
+    // so those modules need the Ktor request/response types on their compile classpath.
+    api(libs.ktor.client.core)
 }