[tintinhamans]

Merge after #657.

The lobby connection indicator was a score from ping and jitter with fixed thresholds, so a link that loses packets, or a peer whose PC is the bottleneck, looked the same as a good one. ConnectionRating (new) turns the measurements of the previous PR into 1 to 5 bars. It is the worse of two axes: the input delay the link forces (from the round trip, input_delay.slack_percent and the send interval) and how often it stalls the game, measured in a match and predicted in the lobby. Four or five bars are green, two or three amber, one red, and red needs a reason (blackout, sustained loss, very high delay or measured stalls). Stalls a healthy link does not explain wait on the peer's frame rate and show as "slow PC", at most 3 bars and never red. More bars only show after the rating asked for them for 3 s.

The lobby slot indicator uses it, and the player info popup now lists ping, packet loss, jitter and the expected input delay instead of "Connection Score" and "Reliability". The loss is the worse direction of the packet feedback over the last minute, from at least 3 reports, and shows "Unknown" before that. F10 toggles a player overlay with the rating and delay per player, Shift+F10 a developer view with the raw stats (ToggleNetworkOverlay(mode) replaces ToggleAdvancedNetworkStats). Texts are rebuilt only when they change and the data is read 4 times a second.

No wire change, no new service key.

Tested: builds x86 Release z_generals. On a slow CPU-bound host in the 3 player rig (full stack) the other clients showed it as "slow PC" in amber instead of red, n=1.