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/ui/auth_screen.cpp

@@ -216,7 +216,9 @@ void AuthScreen::render(auth::AuthHandler& authHandler) {
         if (music) {
             if (!loginMusicVolumeAdjusted_) {
                 savedMusicVolume_ = music->getVolume();
-                int loginVolume = (savedMusicVolume_ * 80) / 100; // reduce auth music by 20%
+                // Reduce music to 80% during login so UI button clicks and error sounds
+                // remain audible over the background track
+                int loginVolume = (savedMusicVolume_ * 80) / 100;
                 if (loginVolume < 0) loginVolume = 0;
                 if (loginVolume > 100) loginVolume = 100;
                 music->setVolume(loginVolume);