feat(gradle-plugin): generate ProGuard/R8 -assumevalues rules for Android/JVM

[kirich1409]

Summary

For each @LocalFlag-annotated ConfigParam with defaultValue = false, generate a ProGuard rule that tells R8 the flag is always disabled, enabling dead branch elimination.

Generated output (proguard-featured.pro)

-assumevalues class dev.androidbroadcast.featured.ConfigValues {
    # Flags frozen at release build time
}

The exact rule shape depends on the final API design for isEnabled — this issue should be implemented after #14.

Acceptance criteria

• Rules file generated during release build task
• File auto-added to Android module's ProGuard configuration
• R8 in a test app eliminates dead code guarded by a @LocalFlag with defaultValue = false
• No rules generated for @RemoteFlag or unannotated flags
• Integration test verifying dead code elimination