Implement NPC greeting voice lines

Added NPC voice manager that plays greeting sounds when clicking on NPCs:

Features:
- Voice line library with multiple race/gender voice types (Human, Dwarf,
  Night Elf, etc.)
- 3D positional audio - voice comes from NPC location
- Cooldown system prevents spam clicking same NPC
- Randomized pitch/volume for variety
- Loads greeting sounds from character voice files in MPQ
- Generic fallback voices for NPCs without specific voice types

Voice lines trigger automatically when gossip window opens (SMSG_GOSSIP_MESSAGE).
Uses same audio system as other sound effects with ma_sound_set_position.

CMakeLists.txt

@@ -104,6 +104,7 @@ set(WOWEE_SOURCES
     src/audio/footstep_manager.cpp
     src/audio/activity_sound_manager.cpp
     src/audio/mount_sound_manager.cpp
+    src/audio/npc_voice_manager.cpp
 
     # Pipeline (asset loaders)
     src/pipeline/mpq_manager.cpp