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/talent_screen.cpp

@@ -76,7 +76,9 @@ void TalentScreen::renderTalentTrees(game::GameHandler& gameHandler) {
         return;
     }
 
-    // Get talent tabs for this class, sorted by orderIndex
+    // Get talent tabs for this class, sorted by orderIndex.
+    // WoW class IDs are 1-indexed (Warrior=1..Druid=11); convert to bitmask for
+    // TalentTab.classMask matching (Warrior=0x1, Paladin=0x2, Hunter=0x4, etc.)
     uint32_t classMask = 1u << (playerClass - 1);
     std::vector<const game::GameHandler::TalentTabEntry*> classTabs;
     for (const auto& [tabId, tab] : gameHandler.getAllTalentTabs()) {