android: register generated sources through the Variant API

Second Android run failed with AGP 9 refusing the previous fix by name:

  You cannot add Provider instances to the Android SourceSet API. [...] Instead
  you should use the Sources interface in the Variant API, in particular
  SourceDirectories.addGeneratedDirectory

AGP cannot tell from a Provider whether the directory holds generated
(read-only) or hand-written (read-write) files, which is a distinction the IDE
needs. `addGeneratedSourceDirectory` is the supported route and — unlike the
plain-path form the error offers as an escape hatch — it carries the task
dependency, so Kotlin still cannot compile before the bindings are generated and
the APK cannot package a stale .so.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -187,25 +187,28 @@ android {
         compose = true
     }
 
-    sourceSets {
-        getByName("main") {
-            // The .so and the bindings are build outputs, not checked-in sources.
-            // Passing the TASK PROVIDERS, not plain paths: Gradle reads each
-            // task's @OutputDirectory and infers the build ordering itself. A
-            // bare path would compile Kotlin before the bindings exist, and the
-            // usual fix — depending on KotlinCompile by type — cannot be written
-            // here, because AGP 9's built-in Kotlin means that class is not on
-            // the buildscript classpath.
-            jniLibs.srcDir(cargoNdkDebug)
-            java.srcDir(generateBindings)
-        }
-    }
-
     packaging {
         resources.excludes += "/META-INF/{AL2.0,LGPL2.1}"
     }
 }
 
+/**
+ * Register the `.so` and the generated bindings as GENERATED sources.
+ *
+ * NOT `sourceSets { ... srcDir(task) }`: AGP 9 rejects a Provider there outright,
+ * because it cannot tell whether the directory holds generated (read-only) or
+ * hand-written (read-write) files — a distinction the IDE needs. The Variant API
+ * is the supported route and, unlike a bare path, `addGeneratedSourceDirectory`
+ * carries the task dependency, so Kotlin cannot compile before the bindings
+ * exist and the APK cannot package a stale `.so`.
+ */
+androidComponents {
+    onVariants { variant ->
+        variant.sources.kotlin?.addGeneratedSourceDirectory(generateBindings, UniffiBindgen::outputDir)
+        variant.sources.jniLibs?.addGeneratedSourceDirectory(cargoNdkDebug, CargoNdkBuild::jniLibsDir)
+    }
+}
+
 dependencies {
     implementation(libs.androidx.core.ktx)
     implementation(libs.androidx.activity.compose)