feat(net): add client-side ping, variance and packet loss natives

[YgorLeal]

Summary

• Adds three new client-side natives: GET_CLIENT_PING, GET_CLIENT_PING_VARIANCE, and GET_CLIENT_PACKET_LOSS
• These values were already available internally via the ENet peer but not exposed to the scripting layer
• Adds GetPacketLoss() to the NetLibrary interface, following the same pattern as GetPing() and GetVariance()

Usage

local ping = GetClientPing()             -- RTT in ms, -1 if not connected
local variance = GetClientPingVariance() -- RTT variance in ms, -1 if not connected
local loss = GetClientPacketLoss()       -- packet loss percentage, -1 if not connected

Test plan

• Connect to a server and verify GetClientPing() returns a valid RTT in ms
• Verify GetClientPingVariance() returns RTT variance matching netgraph 1 display
• Verify GetClientPacketLoss() returns packet loss percentage matching cl_drawPacketLoss
• Verify all three return -1 when not connected to any server

Fixes #2233