Implement SMSG_MULTIPLE_PACKETS unpacking and fix unused variable warning

- Parse bundled sub-packets from SMSG_MULTIPLE_PACKETS using the WotLK
  standard wire format (uint16_be size + uint16_le opcode + payload),
  dispatching each through handlePacket() instead of silently discarding.
  Rate-limited warning for malformed sub-packet overruns.
- Remove unused cullRadiusSq variable in TerrainRenderer::renderShadow()
  that produced a -Wunused-variable warning.

src/rendering/terrain_renderer.cpp

@@ -969,8 +969,6 @@ void TerrainRenderer::renderShadow(VkCommandBuffer cmd, const glm::mat4& lightSp
     vkCmdPushConstants(cmd, shadowPipelineLayout_, VK_SHADER_STAGE_VERTEX_BIT,
                        0, 128, &push);
 
-    const float cullRadiusSq = shadowRadius * shadowRadius;
-
     for (const auto& chunk : chunks) {
         if (!chunk.isValid()) continue;