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/game/packet_parsers_classic.cpp

@@ -282,6 +282,7 @@ bool ClassicPacketParsers::parseMovementBlock(network::Packet& packet, UpdateBlo
             /*uint32_t splineId =*/ packet.readUInt32();
 
             uint32_t pointCount = packet.readUInt32();
+            // Cap waypoints to prevent DoS from malformed packets allocating huge arrays
             if (pointCount > 256) return false;
 
             // points + endPoint (no splineMode in Classic)
@@ -362,7 +363,9 @@ void ClassicPacketParsers::writeMovementPayload(network::Packet& packet, const M
 
     // Transport data (Classic ONTRANSPORT = 0x02000000, no timestamp)
     if (wireFlags & ClassicMoveFlags::ONTRANSPORT) {
-        // Packed transport GUID
+        // Packed GUID compression: only transmit non-zero bytes of the 8-byte GUID.
+        // The mask byte indicates which positions are present (bit N = byte N included).
+        // This is the standard WoW packed GUID wire format across all expansions.
         uint8_t transMask = 0;
         uint8_t transGuidBytes[8];
         int transGuidByteCount = 0;