fix(wireplumber): guard async load callbacks against use-after-free on teardown

The WirePlumber module registers three async callbacks (onDefaultNodesApiLoaded,
onMixerApiLoaded, onPluginActivated) that receive a raw self pointer with a NULL
GCancellable. WirePlumber cannot withdraw an in-flight callback, so if the module
is destroyed before a queued callback fires (e.g. a temporary output/bar is removed
while a component load is still pending, or during an audio route transition), the
callback dereferences the freed self, causing heap corruption / a crash.

Guard each of these callbacks with isModuleAlive(), which checks the existing static
modules registry. The destructor already removes this from the registry before any
teardown, so a missing entry means self is dangling and the callback bails out
without touching it.

A GCancellable cannot fix this cleanly here: every callback dereferences self on its
first line, and wp_core_load_component completes via a WpTransition (not a GTask), so
the cancellable is not recoverable from the GAsyncResult either. The liveness check
must not touch self at all.

Fixes #3974.

include/modules/wireplumber.hpp

@@ -36,6 +36,9 @@ class Wireplumber : public ALabel {
   std::vector<std::string> getWPIcon();
 
   static std::list<waybar::modules::Wireplumber*> modules;
+  // Returns true while `self` is still a live module. Async load/activation callbacks use this to
+  // avoid dereferencing a `self` that was destroyed before the callback fired (see #3974).
+  static bool isModuleAlive(waybar::modules::Wireplumber* self);
 
   uint32_t resolvePhysicalSink(uint32_t start_id);
   uint32_t findPlaybackNodeId(const gchar* description);