feat(android): scaffold the native Android client (Rust-heavy JNI bridge)

Rust-heavy client model (like punktfunk-client-linux): a new cdylib crate
crates/punktfunk-android links punktfunk-core and exposes the JNI seam;
Kotlin (clients/android) owns only the Android-framework surface. Kotlin can't
import the C header the way Swift can, so the bridge is written in Rust to reuse
the Linux client's orchestration rather than re-port it.

- crates/punktfunk-android: JNI bridge — abiVersion/coreVersion native-link
  proof + session connect/close handle; plane pumps stubbed for M4 stage 1.
- clients/android: Gradle project — :app (Compose) + :kit (Android library with
  a cargo-ndk Exec task -> jniLibs). AGP 9.2 / Gradle 9.4.1 / Kotlin 2.3.21 /
  Compose BOM 2026.05.01 / compileSdk 37 / targetSdk 36 / minSdk 31, shipping
  arm64-v8a + x86_64. Phone + TV (leanback) installable. README rewritten.
- .gitea/workflows/android.yml: CI mirroring apple.yml on a Linux runner.
- punktfunk-core: switch rcgen to the ring backend so the whole quic tree is
  aws-lc-free (smaller client .so, cmake-free cross-compile; a win for all targets).

Validated on this box: :app:assembleDebug -> APK with both ABIs; emulator
first-light renders the bridge linked (core ABI v2) with logcat confirmation;
clippy -D warnings + cargo fmt clean; core tests green on the ring backend.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/android/app/src/main/AndroidManifest.xml

@@ -0,0 +1,45 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+
+    <!-- punktfunk/1 QUIC/UDP data plane. -->
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <!-- mDNS discovery of _punktfunk._udp on the LAN (NsdManager). -->
+    <uses-permission
+        android:name="android.permission.NEARBY_WIFI_DEVICES"
+        android:usesPermissionFlags="neverForLocation" />
+    <!-- Enforced from Android 17 (SDK 37) for ALL local-network traffic incl. the QUIC socket.
+         Harmless to declare on earlier releases. -->
+    <uses-permission android:name="android.permission.ACCESS_LOCAL_NETWORK" />
+    <!-- Mic uplink to the host's virtual microphone (requested at runtime). -->
+    <uses-permission android:name="android.permission.RECORD_AUDIO" />
+    <!-- Gamepad rumble feedback. -->
+    <uses-permission android:name="android.permission.VIBRATE" />
+
+    <!-- We target phone + TV from day one: keep the app installable on TV (no touchscreen) and on
+         devices without a gamepad. -->
+    <uses-feature android:name="android.hardware.touchscreen" android:required="false" />
+    <uses-feature android:name="android.software.leanback" android:required="false" />
+    <uses-feature android:name="android.hardware.gamepad" android:required="false" />
+
+    <application
+        android:allowBackup="true"
+        android:icon="@android:drawable/sym_def_app_icon"
+        android:label="@string/app_name"
+        android:supportsRtl="true"
+        android:theme="@style/Theme.PunktfunkAndroid">
+
+        <activity
+            android:name=".MainActivity"
+            android:exported="true"
+            android:configChanges="orientation|screenSize|keyboardHidden|screenLayout|density|navigation"
+            android:theme="@style/Theme.PunktfunkAndroid">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.LAUNCHER" />
+                <!-- TV launcher entry. -->
+                <category android:name="android.intent.category.LEANBACK_LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+</manifest>

clients/android/app/src/main/kotlin/io/unom/punktfunk/MainActivity.kt

@@ -0,0 +1,59 @@
+package io.unom.punktfunk
+
+import android.os.Bundle
+import android.util.Log
+import androidx.activity.ComponentActivity
+import androidx.activity.compose.setContent
+import androidx.activity.enableEdgeToEdge
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.material3.darkColorScheme
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import io.unom.punktfunk.kit.NativeBridge
+
+class MainActivity : ComponentActivity() {
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+
+        // Cross the JNI bridge into libpunktfunk_android.so → punktfunk-core. A live ABI version is
+        // the scaffold's proof the whole native stack is wired (cargo-ndk → jniLibs → APK →
+        // System.loadLibrary → JNI → core). Logged so it's verifiable headlessly via logcat.
+        val abi = runCatching { NativeBridge.abiVersion() }.getOrDefault(-1)
+        val core = runCatching { NativeBridge.coreVersion() }.getOrDefault("?")
+        Log.i("punktfunk", "native bridge: core ABI v$abi, core $core")
+
+        enableEdgeToEdge()
+        setContent {
+            MaterialTheme(colorScheme = darkColorScheme()) {
+                Surface(modifier = Modifier.fillMaxSize()) {
+                    ScaffoldScreen(abi, core)
+                }
+            }
+        }
+    }
+}
+
+@Composable
+private fun ScaffoldScreen(abi: Int, core: String) {
+    Column(
+        modifier = Modifier.fillMaxSize().padding(24.dp),
+        horizontalAlignment = Alignment.CenterHorizontally,
+        verticalArrangement = Arrangement.Center,
+    ) {
+        Text("punktfunk", style = MaterialTheme.typography.headlineMedium)
+        Text("Android client — scaffold", style = MaterialTheme.typography.bodyMedium)
+        Text(
+            if (abi > 0) "✓ native bridge linked" else "✗ native bridge FAILED",
+            style = MaterialTheme.typography.titleMedium,
+        )
+        Text("core ABI v$abi · core $core", style = MaterialTheme.typography.bodySmall)
+    }
+}

clients/android/app/src/main/res/values/strings.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <string name="app_name">punktfunk</string>
+</resources>

clients/android/app/src/main/res/values/themes.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <!-- The Activity is pure Compose; this platform theme just provides a no-action-bar host.
+         Compose draws its own Material 3 surfaces over it. -->
+    <style name="Theme.PunktfunkAndroid" parent="android:Theme.Material.NoActionBar" />
+</resources>