Add session_jitter and period_jitter to the display_stats line in SecondLife.log

[Dan-Linden]

A log line like this is added to SecondLife.log every minute:
2025-10-17T14:38:48Z INFO # newview/llviewerdisplay.cpp(244) display_stats : FPS: 21.80
For purposes of performance testing, I'd like session_jitter and period_jitter also logged every minute. Logging it to some other line than display_stats is acceptable.

session_jitter is the "normalized sess. jitter" and period_jitter is the "normalized period jitter" in the Statistics floater.

Test Plan

Verify session_jitter and period_jitter is logged once per minute in SecondLife.log.
Verify they match "normalized sess. jitter" and "normalized period jitter" from the Statistics floater.

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