feat: scaffold featured-gradle-plugin module

[kirich1409]

Summary

Create the featured-gradle-plugin Gradle plugin module that will generate dead-code-elimination artifacts for release builds.

Scope

• New module featured-gradle-plugin/ at repo root
• Registers a Gradle plugin with ID dev.androidbroadcast.featured
• Plugin applied to app/library modules via build.gradle.kts
• Wired into the release build task graph (runs before minification)
• Added to settings.gradle.kts and published as a separate artifact

Acceptance criteria

• Module created with correct build.gradle.kts (Kotlin DSL, Java 21)
• Plugin class registered in META-INF/gradle-plugins/
• Plugin applies without error on sample app
• Added to settings.gradle.kts