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/build.gradle.kts

@@ -5,4 +5,8 @@
 plugins {
     alias(libs.plugins.kotlin.jvm) apply false
     alias(libs.plugins.kotlin.serialization) apply false
+    // AGP 9.2.1 — proven to build against Android SDK 35 with Gradle 9.6.1.
+    // AGP 9 has built-in Kotlin; framework modules apply only these (no kotlin.android).
+    alias(libs.plugins.android.library) apply false
+    alias(libs.plugins.android.application) apply false
 }