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/charge_effect.cpp

@@ -474,11 +474,13 @@ void ChargeEffect::emit(const glm::vec3& position, const glm::vec3& direction) {
     // Spawn dust puffs at feet
     glm::vec3 horizDir = glm::vec3(direction.x, direction.y, 0.0f);
     float horizLenSq = glm::dot(horizDir, horizDir);
+    // Skip dust when character is nearly stationary — prevents NaN from inversesqrt(0)
     if (horizLenSq < 1e-6f) return;
     float invHorizLen = glm::inversesqrt(horizLenSq);
     glm::vec3 backDir = -horizDir * invHorizLen;
     glm::vec3 sideDir = glm::vec3(-backDir.y, backDir.x, 0.0f);
 
+    // Accumulate ~0.48 per frame at 60fps (30 particles/sec * 16ms); emit when >= 1.0
     dustAccum_ += 30.0f * 0.016f;
     while (dustAccum_ >= 1.0f && dustPuffs_.size() < MAX_DUST) {
         dustAccum_ -= 1.0f;