docs: add why-comments to rendering, packets, and UI code

- charge_effect: explain inversesqrt guard (prevents NaN on stationary
  character) and dust accumulator rate (30 particles/sec * 16ms)
- swim_effects: explain why insect pipeline disables depth test
  (screen-space sprites must render above water geometry)
- packet_parsers_classic: explain spline waypoint cap (DoS prevention)
  and packed GUID compression format (non-zero bytes only, mask byte)
- talent_screen: explain class ID to bitmask conversion (1-indexed
  WoW class IDs → power-of-2 mask for TalentTab.classMask matching)
- auth_screen: explain login music volume reduction (80% so UI sounds
  remain audible over background track)

src/rendering/swim_effects.cpp

@@ -175,6 +175,8 @@ bool SwimEffects::initialize(VkContext* ctx, VkDescriptorSetLayout perFrameLayou
             return false;
         }
 
+        // Depth test disabled — insects are screen-space sprites that must always
+        // render above the water surface regardless of scene geometry.
         insectPipeline = PipelineBuilder()
             .setShaders(vertStage, fragStage)
             .setVertexInput({binding}, attrs)