docs: add current structure/implementation/design documentation

docs/README.md

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+# iperf3-gui
+
+A Flutter-based Android GUI wrapper for iperf3, the popular network performance measurement tool.
+
+## Architecture
+
+```
+lib/                          # Flutter/Dart application layer
+├── main.dart                 # App entry point (MaterialApp)
+├── domain/                   # Business logic (platform-independent)
+│   ├── models/
+│   │   ├── test_configuration.dart   # Server, port, duration, interval, parallel streams
+│   │   └── iperf3_result.dart        # Raw output + exit code
+│   └── services/
+│       ├── iperf3_service.dart       # Dart → MethodChannel bridge (async with Completer)
+│       └── iperf3_binary_path_provider.dart   # Deprecated (kept for reference)
+└── ui/                       # Presentation layer
+    └── features/
+        └── test_run/
+            ├── test_run_screen.dart          # Server/port inputs + GO button
+            └── test_run_view_model.dart      # State machine: idle → running → completed/error
+
+android/app/src/main/
+├── kotlin/                     # Kotlin plugin layer
+│   └── com/novoyuuparosk/iperf3gui/
+│       ├── MainActivity.kt     # Registers IperfRunner as FlutterPlugin
+│       └── IperfRunner.kt      # MethodChannel handler → native JNI calls
+└── cpp/                        # Native iperf3 library
+    ├── CMakeLists.txt          # Compiles iperf3 + cJSON + JNI into libiperf3jni.so
+    ├── iperf_jni.c             # JNI bridge: Java String[] → iperf3 API
+    ├── iperf/                  # iperf3 3.19 C source tree (see below)
+    ├── iperf_config.h          # Generated Android-compatible config header
+    └── iperf_config_android.h  # Android-specific compile options
+```
+
+## Data Flow
+
+```
+User taps GO
+  → Dart TestRunViewModel.runTest(config)
+  → Iperf3Service.run(config) via MethodChannel
+  → Kotlin IperfRunner.runIperfLive(args, callback)
+  → JNI C: java_string_array_to_argv() → iperf_parse_arguments()
+  → iperf_run_client(test)
+  → Reader thread pipes stdout → CallVoidMethod → Dart onOutput callbacks
+  → On completion: Dart Completer resolves with Iperf3Result
+```
+
+## Key Design Decisions
+
+### JNI Over Process Execution
+Android SELinux Enforcing mode blocks `execve()` from the `untrusted_app` domain, making Process.start() of external binaries fail with Permission denied. Compiling iperf3 as a shared library linked into the app process eliminates this restriction entirely — iperf3 code runs in-process with no domain transitions needed.
+
+### Callback Mechanism
+The JNI bridge uses `CallVoidMethod` on stored `jmethodID`s to invoke the Kotlin `IperfCallback` interface, NOT function pointer casts. Direct function pointer casts caused SIGBUS crashes on ARM due to JNINativeInterface method ID layout differences.
+
+### Output Streaming
+iperf3 output is redirected via pipe to a background reader thread that attaches to the Java VM via `AttachCurrentThread` and invokes callbacks via `CallVoidMethod`. The reader thread frees the state struct on exit; the JNI caller owns the callback object's lifetime.
+
+### Async Pattern
+Dart uses `Completer<Iperf3Result>` to await native completion. The MethodChannel callback handler resolves the completer on `onComplete`, with a timeout fallback that calls `stopIperf` on the native layer.
+
+## iperf3 Source
+
+- Version: 3.19
+- Location: `android/app/src/main/cpp/iperf/`
+- Files: Full C source tree including cJSON, headers, man pages, PEM keys
+- Build: CMake compiles into `libiperf3jni.so` for `armeabi-v7a` and `arm64-v8a`
+- Config: `iperf_config.h` generated from `iperf_config.h.in` with Android-compatible POSIX/threading macros
+
+## Native JNI API
+
+| Kotlin Method | JNI Signature | Purpose |
+|---|---|---|
+| `runIperfLive(String[], IperfCallback)` | `runIperfLive` | Start test with args and callback |
+| `forceStopIperfTest(IperfCallback)` | `forceStopIperfTest` | Graceful stop via shutdown() |
+| `isTestRunning()` | `isTestRunning` | Check if test is in progress |
+
+## Building
+
+Prerequisites:
+- Flutter 3.47+
+- Android SDK with platform android-36
+- JDK 17
+- NDK r28+
+- CMake 3.22.1
+
+```bash
+flutter pub get
+flutter build apk --debug
+adb install build/app/outputs/flutter-apk/app-debug.apk
+```
+
+## Dependencies
+
+- Dart: `provider`, `path_provider`
+- Android: CMake external native build
+- Native: iperf3 3.19, cJSON (single file)
+
+## License
+
+MIT — bundled iperf3 3.19 is BSD-3-Clause licensed.