F-116: sample domain use cases; kill GlobalScope in list paging

[stepango]

Summary

Closes GH #48 / F-116: gold sample clean architecture for characters list/detail.

• Domain use cases on feature/characters/core (IGetCharactersUseCase / IGetCharacterUseCase + impls)
• Exposed via CharactersCoreFeature (same Dagger component-dependency pattern as favorite)
• List paging DataSource is a thin adapter: use case + CoroutineScope from viewModelScope — no GlobalScope, no direct MarvelRepository
• Detail VM uses IGetCharacterUseCase instead of repository
• Docs: docs/SAMPLE-APP.md characters layering section

Verify (host)

cd application && ./gradlew :feature-characters-list-impl:compileDebugKotlin   :feature-characters-detail-impl:compileDebugKotlin   :feature-characters-core-impl:compileDebugKotlin   :binary:assembleDebug
# BUILD SUCCESSFUL
grep -R GlobalScope application --include='*.kt'  # empty

Matrix / axioms

• No impl→impl, no androidLibrary, no Forma engine changes
• Presentation depends on use-case interfaces from core.api only

Closes #48