chore(android): install Android SDK + wire AGP 9.2.1 (toolchain proven)

Android SDK installed on this machine (cmdline-tools via brew;
platform-tools + platforms;android-35 + build-tools;35.0.0), local.properties
points Gradle at it (gitignored). Wired AGP 9.2.1 into the version catalog +
google() repos; proved the full chain by building a throwaway android-library
module against SDK 35 (AAR produced), then removed the probe.

Groundwork for AW2+: AGP plugins in the catalog, google() in settings, and the
working recipe in README (incl. the AGP-9-has-built-in-Kotlin gotcha — Android
modules apply ONLY the android plugin, never kotlin.android). Pure JVM modules
still green (218 tests). Framework module stubs stay gated until implemented.

android/gradle/libs.versions.toml

@@ -4,6 +4,7 @@
 ## compatibility with the Kotlin Gradle Plugin on Gradle 9.6).
 
 [versions]
+agp = "9.2.1"
 kotlin = "2.3.21"
 kotlinxSerialization = "1.9.0"
 kotlinxCoroutines = "1.10.2"
@@ -35,3 +36,7 @@ unit-test = ["junit-jupiter", "kotlinx-coroutines-test", "turbine", "mockk"]
 [plugins]
 kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
 kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }
+android-library = { id = "com.android.library", version.ref = "agp" }
+android-application = { id = "com.android.application", version.ref = "agp" }
+# NOTE: AGP 9+ has BUILT-IN Kotlin support — Android modules apply only the
+# android plugin, NOT a separate org.jetbrains.kotlin.android (it errors out).