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/build.gradle.kts

@@ -0,0 +1,67 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
+plugins {
+    id("com.android.application")
+    // AGP 9 built-in Kotlin: NO org.jetbrains.kotlin.android. The Compose compiler plugin is
+    // supplied by AGP, so it's applied without a version.
+    id("org.jetbrains.kotlin.plugin.compose")
+}
+
+android {
+    namespace = "io.unom.punktfunk"
+    compileSdk = 37 // Android 17 — required by androidx.core 1.19.0; targetSdk stays 36 for now.
+
+    defaultConfig {
+        applicationId = "io.unom.punktfunk"
+        minSdk = 31
+        targetSdk = 36
+        versionCode = 1
+        versionName = "0.0.1"
+        ndk { abiFilters += listOf("arm64-v8a", "x86_64") }
+    }
+
+    buildTypes {
+        release {
+            isMinifyEnabled = false // scaffold; enable R8 + shrinkResources later
+        }
+    }
+
+    buildFeatures { compose = true }
+
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_21
+        targetCompatibility = JavaVersion.VERSION_21
+    }
+    packaging {
+        jniLibs {
+            useLegacyPackaging = false
+            // punktfunk-core is statically linked into libpunktfunk_android.so (rlib). Its standalone
+            // cdylib (built because the core crate also declares crate-type = cdylib) is never loaded
+            // by Kotlin — drop it from the APK rather than ship ~5–9 MB of dead code.
+            excludes += "**/libpunktfunk_core.so"
+        }
+    }
+}
+
+kotlin { compilerOptions { jvmTarget.set(JvmTarget.JVM_21) } }
+
+dependencies {
+    implementation(project(":kit"))
+
+    val composeBom = platform("androidx.compose:compose-bom:2026.05.01")
+    implementation(composeBom)
+
+    implementation("androidx.core:core-ktx:1.19.0")
+    implementation("androidx.activity:activity-compose:1.13.0")
+    implementation("androidx.lifecycle:lifecycle-runtime-ktx:2.10.0")
+
+    implementation("androidx.compose.ui:ui")
+    implementation("androidx.compose.ui:ui-tooling-preview")
+    implementation("androidx.compose.foundation:foundation")
+    implementation("androidx.compose.material3:material3")
+    debugImplementation("androidx.compose.ui:ui-tooling")
+
+    // Android TV components (we target phone + TV) land in the TV-UI milestone:
+    //   implementation("androidx.tv:tv-material:1.1.0")
+    // The manifest already declares leanback so the scaffold installs on TV.
+}