docs: add BRANCH-android-rewrite.md and update ARCH/ADMIN/USER_GUIDE
Documents the android-rewrite branch story end-to-end:
- Why the Kotlin+JNI stack was abandoned (stack overflow, libcrypto
TLS race, __init_tcb TCB leak, ring runtime reuse crash)
- The Tauri 2.x Mobile pivot that reuses the desktop codebase verbatim
- Android-specific pieces: wzp-native standalone cdylib loaded via
libloading, android_audio.rs JVM routing, Oboe audio config quirks
- Build pipeline via build-tauri-android.sh + wzp-android-builder image
- Known quirks (API 34/36 coexistence, NDK path absolutes, etc.)
Also appends shared-doc sections (identical on both branches):
- ARCHITECTURE.md: "Audio Backend Architecture (Platform Matrix)"
covering CPAL / VPIO / WASAPI / Oboe backends, selection matrix,
the wzp-native cdylib rationale, and the vendored audiopus_sys fix.
- ADMINISTRATION.md: "Build Pipelines" with Docker images
(wzp-android-builder, wzp-windows-builder), per-pipeline usage
(Android APK, Linux x86_64, Windows .exe), the Hetzner Cloud
alternative, ntfy/rustypaste integration, and credential locations.
- USER_GUIDE.md: "Direct 1:1 Calling (Desktop + Android)" covering
history + recent contacts + deregister UI, and "Windows AEC
Variants" explaining the AEC vs noAEC builds and driver caveats.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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