Migrate a Flutter Android project to AGP 9 with built-in Kotlin
The full path from an AGP 8 Flutter app that applies kotlin-android to AGP 9.1 with android.builtInKotlin=true on Flutter 3.47. Every step was built and measured, including the two edits that look optional and are not: kotlinOptions is a compile error under KGP 2.2+, and the KGP line in settings.gradle.kts has to stay.