CellGuard app: on-device IMSI-catcher detection (Kotlin/Compose)

- foreground DetectorService: cgcap capture + on-device decode
- framework CellInfo source: real serving+neighbor towers, signal, carrier name
- FrameDecoder: 0x070F cell-info SIT decode
- block-2G/3G + detection toggles via cgctl; libsu root bridge
- silent status notification + edge-triggered alerts

BUILD.md

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+# Building CellGuard
+
+## Toolchain
+
+| Component         | Version   |
+|-------------------|-----------|
+| Gradle (wrapper)  | 8.9       |
+| Android Gradle Plugin | 8.5.2 |
+| Kotlin            | 1.9.23    |
+| Compose compiler  | 1.5.11    |
+| JDK               | 21        |
+| compileSdk / targetSdk | 35   |
+| minSdk            | 33        |
+| Build tools       | 35.0.0    |
+
+These versions are mutually compatible: AGP 8.5.2 requires Gradle 8.7+ and JDK 17+
+(JDK 21 is fine), and Kotlin 1.9.23 pins the Compose compiler extension to 1.5.11.
+
+## Prerequisites
+
+- JDK 21 installed at `/usr/lib/jvm/java-21-openjdk-arm64`
+- Android SDK at `/home/snake/Android/Sdk` with:
+  - `platforms/android-35`
+  - `build-tools/35.0.0`
+- Network access for the first build (downloads AndroidX, Compose BOM 2024.06.00,
+  and libsu 5.2.2 from Google, Maven Central, and JitPack).
+
+## Build command (debug APK)
+
+```bash
+export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-arm64
+export ANDROID_HOME=/home/snake/Android/Sdk
+cd /home/snake/CellGuard-app && ./gradlew :app:assembleDebug
+```
+
+## Output
+
+The debug APK is written to:
+
+```
+/home/snake/CellGuard-app/app/build/outputs/apk/debug/app-debug.apk
+```
+
+For a release (unsigned) build, run `./gradlew :app:assembleRelease`; the artifact
+lands in `app/build/outputs/apk/release/`.