fix: crash on launch — don't auto-start call, handle null JNI strings, remove stdout tracing

- CallActivity no longer auto-starts a call on launch
- CallViewModel lazily inits engine only when startCall() is called
- nativeGetStats nullable return handled safely in Kotlin
- Removed tracing_subscriber::fmt() which panics on Android (no stdout)
- All JNI calls wrapped in try/catch on Kotlin side

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

crates/wzp-android/src/jni_bridge.rs

@@ -109,13 +109,9 @@ pub unsafe extern "system" fn Java_com_wzp_engine_WzpEngine_nativeInit(
     _class: *mut c_void,
 ) -> JLong {
     let result = panic::catch_unwind(|| {
-        // Initialise tracing once (ignore errors if already set).
-        #[cfg(target_os = "android")]
-        {
-            let _ = tracing_subscriber::fmt()
-                .with_max_level(tracing::Level::INFO)
-                .try_init();
-        }
+        // Note: tracing on Android requires android_logger or similar.
+        // fmt() subscriber writes to stdout which doesn't exist on Android.
+        // Skip tracing init here — add android_logger later.
 
         let handle = Box::new(EngineHandle {
             engine: WzpEngine::new(),