fix: eliminate all native thread creation — run everything single-threaded

pthread_create crashes on Android due to static bionic __init_tcb stubs
in the Rust std prebuilt rlibs. This is unfixable without rebuilding std.

Solution: run the entire call (QUIC connect, handshake, media send/recv)
on a single tokio current_thread runtime. The JNI startCall() now blocks,
so Kotlin dispatches it to Dispatchers.IO (JVM thread, not pthread).

Audio pipeline temporarily simplified to silence frames — will restore
once threading is solved (either via Java Thread or rebuilding std).

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

android/app/src/main/java/com/wzp/ui/call/CallViewModel.kt

@@ -56,12 +56,22 @@ class CallViewModel : ViewModel(), WzpCallback {
                 engineInitialized = true
             }
             _callState.value = 1 // Connecting
-            val result = engine?.startCall(relayAddr, room) ?: -1
-            if (result == 0) {
-                startStatsPolling()
-            } else {
-                _callState.value = 0
-                _errorMessage.value = "Failed to start call (code $result)"
+            startStatsPolling()
+
+            // startCall blocks (runs tokio on calling thread), so dispatch
+            // to a background coroutine. Using Dispatchers.IO which uses
+            // Java threads (not native pthread_create).
+            viewModelScope.launch(kotlinx.coroutines.Dispatchers.IO) {
+                try {
+                    val result = engine?.startCall(relayAddr, room) ?: -1
+                    if (result != 0) {
+                        _callState.value = 0
+                        _errorMessage.value = "Failed to start call (code $result)"
+                    }
+                } catch (e: Exception) {
+                    _callState.value = 0
+                    _errorMessage.value = "Engine error: ${e.message}"
+                }
             }
         } catch (e: Exception) {
             _callState.value = 0