docs: add why-comments to TBC parsers, bell audio, portrait preview

- packet_parsers_tbc: explain spline waypoint cap (DoS prevention),
  spline compression flags (Catmull-Rom 0x80000 / linear 0x2000 use
  uncompressed format, others use packed delta), spell hit target cap
  (128 >> real AOE max of ~20), guild roster cap (1000 safety limit)
- ambient_sound_manager: explain 1.5s bell toll spacing — matches
  retail WoW cadence, allows each toll to ring out before the next
- character_preview.hpp: explain 4:5 portrait aspect ratio for
  full-body character display in creation/selection screen

include/rendering/character_preview.hpp

@@ -81,6 +81,8 @@ private:
     // ImGui texture handle for displaying the preview (VkDescriptorSet in Vulkan backend)
     VkDescriptorSet imguiTextureId_ = VK_NULL_HANDLE;
 
+    // 4:5 portrait aspect ratio — taller than wide to show full character body
+    // from head to feet in the character creation/selection screen
     static constexpr int fboWidth_ = 400;
     static constexpr int fboHeight_ = 500;