Scaffold native Android build and enforce Rust boundaries (#89)

docs/android/architecture.md

@@ -79,6 +79,10 @@ At #89 pin a mutually supported stable AGP/Gradle/Kotlin/KSP/Hilt set, JDK,
 SDK build tools and NDK, plus wrapper checksum. The current [AGP 9.4 contract](https://developer.android.com/build/releases/agp-9-4-0-release-notes)
 requires Gradle 9.6.0 and JDK 17 and supports API 37. This is a candidate build
 baseline, subject to actual artifact availability and plugin compatibility.
+The #89 scaffold pins AGP 9.4.1, Gradle 9.8.1, Kotlin 2.4.20, KSP 2.3.12,
+Hilt 2.60.1 and JDK 25 (JVM output 17); its catalog and build instructions live
+in [android/README.md](../../android/README.md). These pins are verified by the
+scaffold build gates rather than inferred from minimum-version requirements.
 No dynamic versions, snapshots or automatic toolchain substitutions. #92 pins
 cargo-ndk, Rust targets/toolchain and the workspace UniFFI version and builds all
 ABIs from the locked Cargo graph. Retain separate native symbols and R8 mappings.