audio: stop precast sound on spell completion, failure, or interrupt

Add AudioEngine::playSound2DStoppable() + stopSound() so callers can
hold a handle and cancel playback early. SpellSoundManager::playPrecast()
now stores the handle in activePrecastId_; stopPrecast() cuts the sound.

playCast() calls stopPrecast() before playing the release sound, so the
channeling audio never bleeds past cast time. SMSG_SPELL_FAILURE and
SMSG_CAST_FAILED both call stopPrecast() so interrupted casts silence
immediately.

include/audio/audio_engine.hpp

@@ -45,6 +45,11 @@ public:
     bool playSound2D(const std::vector<uint8_t>& wavData, float volume = 1.0f, float pitch = 1.0f);
     bool playSound2D(const std::string& mpqPath, float volume = 1.0f, float pitch = 1.0f);
 
+    // Stoppable 2D sound — returns a non-zero handle, or 0 on failure
+    uint32_t playSound2DStoppable(const std::vector<uint8_t>& wavData, float volume = 1.0f);
+    // Stop a sound started with playSound2DStoppable (no-op if already finished)
+    void stopSound(uint32_t id);
+
     // 3D positional sound playback
     bool playSound3D(const std::vector<uint8_t>& wavData, const glm::vec3& position,
                      float volume = 1.0f, float pitch = 1.0f, float maxDistance = 100.0f);
@@ -70,8 +75,10 @@ private:
         ma_sound* sound;
         void* buffer;  // ma_audio_buffer* - Keep audio buffer alive
         std::shared_ptr<const std::vector<uint8_t>> pcmDataRef;  // Keep decoded PCM alive
+        uint32_t id = 0;  // 0 = anonymous (not stoppable)
     };
     std::vector<ActiveSound> activeSounds_;
+    uint32_t nextSoundId_ = 1;
 
     // Music track state
     ma_sound* musicSound_ = nullptr;

include/audio/spell_sound_manager.hpp

@@ -45,6 +45,7 @@ public:
 
     // Spell casting sounds
     void playPrecast(MagicSchool school, SpellPower power);  // Channeling/preparation
+    void stopPrecast();                                       // Stop precast sound early
     void playCast(MagicSchool school);                        // When spell fires
     void playImpact(MagicSchool school, SpellPower power);    // When spell hits target
 
@@ -96,6 +97,7 @@ private:
     // State tracking
     float volumeScale_ = 1.0f;
     bool initialized_ = false;
+    uint32_t activePrecastId_ = 0;  // Handle from AudioEngine::playSound2DStoppable()
 
     // Helper methods
     bool loadSound(const std::string& path, SpellSample& sample, pipeline::AssetManager* assets);