Add native Android app scaffold (M0): whisper.cpp engine via JNI

- android/: Gradle/Kotlin project (AGP 9.4, Compose, NDK 27.1), monorepo
  subdir as planned; whisper.cpp v1.9.3 vendored via pinned fetch script
- core/whisper: JNI wrapper (beam size, threads, language) + WhisperEngine
  Kotlin API mirroring the desktop engine contract
- app (M0 scope): in-app GGML model download from Hugging Face, engine
  load, WAV picker with resampling, on-device transcription, share sheet
- build validated: assembleDebug OK, libwhisper_jni.so + ggml packaged

android/build.gradle.kts

@@ -0,0 +1,6 @@
+// Root build file — plugin versions come from gradle/libs.versions.toml.
+plugins {
+    alias(libs.plugins.android.application) apply false
+    alias(libs.plugins.android.library) apply false
+    alias(libs.plugins.kotlin.compose) apply false
+}