fix: link libc++ statically — crash on launch due to missing libc++_shared.so

The app crashed immediately when loading libwzp_android.so because the
cc crate's default dynamic linking produced a runtime dependency on
libc++_shared.so, which was never packaged into the APK.

Adding .cpp_link_stdlib(Some("c++_static")) to build.rs bakes the C++
runtime into libwzp_android.so directly, eliminating the missing .so.

See issues/001-libc++-shared-crash.md for full diagnosis and logcat trace.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-android/build.rs

@@ -13,6 +13,7 @@ fn main() {
                 cc::Build::new()
                     .cpp(true)
                     .std("c++17")
+                    .cpp_link_stdlib(Some("c++_static"))
                     .file("cpp/oboe_bridge.cpp")
                     .include("cpp")
                     .include(oboe_path.join("include"))
@@ -24,6 +25,7 @@ fn main() {
                 cc::Build::new()
                     .cpp(true)
                     .std("c++17")
+                    .cpp_link_stdlib(Some("c++_static"))
                     .file("cpp/oboe_stub.cpp")
                     .include("cpp")
                     .compile("oboe_bridge");

issues/001-libc++-shared-crash.md

@@ -0,0 +1,89 @@
+# Issue 001: App crashes on launch — missing libc++_shared.so
+
+## Status: Fix committed, needs rebuild
+
+## Symptom
+
+App opens, shows splash screen, then immediately crashes back to home screen.
+Crash occurs when user taps CALL (which triggers `System.loadLibrary("wzp_android")`),
+or on any code path that first loads the native library.
+
+## Device tested
+
+- Nothing Phone, arm64-v8a, Android 15
+- ADB device ID: `00142151B000973`
+
+## Logcat crash trace
+
+```
+E AndroidRuntime: FATAL EXCEPTION: main
+E AndroidRuntime: Process: com.wzp.phone, PID: 6048
+E AndroidRuntime: java.lang.UnsatisfiedLinkError: dlopen failed: library "libc++_shared.so"
+    not found: needed by /data/app/.../base.apk!/lib/arm64-v8a/libwzp_android.so
+    in namespace clns-9
+E AndroidRuntime:   at java.lang.Runtime.loadLibrary0(Runtime.java:1097)
+E AndroidRuntime:   at java.lang.System.loadLibrary(System.java:1765)
+E AndroidRuntime:   at com.wzp.engine.WzpEngine.<clinit>(WzpEngine.kt:115)
+E AndroidRuntime:   at com.wzp.ui.call.CallViewModel.startCall(CallViewModel.kt:52)
+E AndroidRuntime:   at com.wzp.ui.call.InCallScreenKt$InCallScreen$1$1$1.invoke(InCallScreen.kt:96)
+```
+
+## Root cause
+
+`crates/wzp-android/build.rs` uses the `cc` crate to compile C++17 code
+(the Oboe audio bridge). On Android targets, `cc::Build` defaults to dynamically
+linking the C++ standard library (`libc++_shared.so`).
+
+This means `libwzp_android.so` has a runtime dependency on `libc++_shared.so`.
+However, the Gradle build (`cargoNdkBuild` task) only copies `libwzp_android.so`
+into `jniLibs/arm64-v8a/`. The C++ runtime is never copied alongside it, so the
+APK ships without `libc++_shared.so`.
+
+At runtime, `dlopen("libwzp_android.so")` fails because the linker can't find
+the missing shared library in the app's namespace.
+
+### Why this wasn't caught earlier
+
+The previous APK (pre-QUIC wiring) was 2.0MB release / 8.9MB debug. The Oboe
+C++ bridge was likely being compiled but the native library may not have been
+loaded on the code path that was tested, or the dependency was satisfied by a
+different linking configuration at the time.
+
+## Fix
+
+In `crates/wzp-android/build.rs`, add `.cpp_link_stdlib(Some("c++_static"))` to
+all `cc::Build` invocations targeting Android. This tells the `cc` crate to link
+`libc++_static.a` instead of `libc++_shared.so`, baking the C++ runtime directly
+into `libwzp_android.so`. No separate shared library needed at runtime.
+
+```rust
+cc::Build::new()
+    .cpp(true)
+    .std("c++17")
+    .cpp_link_stdlib(Some("c++_static"))  // <-- this line
+    .file("cpp/oboe_bridge.cpp")
+    // ...
+```
+
+Applied to both the Oboe build path and the stub fallback path.
+
+### Trade-off
+
+Static linking increases `libwzp_android.so` size slightly (~200-400KB for
+libc++ static). This is acceptable — the alternative (bundling libc++_shared.so
+separately) adds complexity to the Gradle build and risks version mismatches if
+multiple native libraries each bundle their own shared copy.
+
+## Rebuild steps
+
+```bash
+cd android && ./gradlew assembleRelease
+adb install -r app/build/outputs/apk/release/app-release.apk
+```
+
+## Verification
+
+After install, the app should:
+1. Open without crashing
+2. Load `libwzp_android.so` successfully (check logcat for absence of UnsatisfiedLinkError)
+3. Show the call UI with CALL button