test(gradle-plugin): integration tests for ProGuard rule generation and R8 DCE

[kirich1409]

Summary

Verify that the Gradle plugin correctly generates ProGuard rules and that R8 actually eliminates dead code in the output APK/JAR.

Test cases

1. @LocalFlag with defaultValue = false → rule generated, dead branch removed from release APK
2. @LocalFlag with defaultValue = true → rule generated, dead branch of the else removed
3. @RemoteFlag → no rule generated, code present in both debug and release
4. Unannotated flag → no rule generated
5. Multi-module: flags from 3 modules all generate rules correctly

Acceptance criteria

• Tests run as part of ./gradlew :featured-gradle-plugin:test
• Use TestKit (Gradle TestKit) for plugin integration tests
• APK diff validates dead code elimination