Use weak symbol for haskellUseScrollDemo in jni_bridge.c

The useScrollDemo JNI method was added as test infrastructure for
the ScrollView integration tests. It calls haskellUseScrollDemo(),
which was declared extern — forcing every app using haskell-mobile
as a library to export that symbol from Haskell.

Apps that replace HaskellMobile/App.hs (via extraModuleCopy) and
don't provide a scroll demo would get an UnsatisfiedLinkError at
startup when Android's linker tried to load libprrrrrrrrr.so.

Fix: define haskellUseScrollDemo with __attribute__((weak)) so it
acts as a no-op fallback. Apps that do want scroll-demo behaviour
can override it with a 'foreign export ccall haskellUseScrollDemo'
in their Haskell source; apps that don't need it link cleanly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jappeace-sloth

cbits/jni_bridge.c

@@ -24,7 +24,12 @@ extern void haskellOnLifecycle(void *ctx, int eventType);
 extern void haskellRenderUI(void *ctx);
 extern void haskellOnUIEvent(void *ctx, int callbackId);
 extern void haskellOnUITextChange(void *ctx, int callbackId, const char *text);
-extern void haskellUseScrollDemo(void);
+
+/* Weak default for apps that don't provide a scroll-demo.
+ * Apps that want useScrollDemo to do something meaningful should define
+ * haskellUseScrollDemo via 'foreign export ccall' in their Haskell source,
+ * which overrides this no-op at link time. */
+void __attribute__((weak)) haskellUseScrollDemo(void) {}
 
 /* Android UI bridge (from ui_bridge_android.c) */
 extern void setup_android_ui_bridge(JNIEnv *env, jobject activity, void *haskellCtx);