load tests: show elapsed, average and ETA in the live stats

The script-run head and the suite summary now display, while a load run
repeats: the elapsed time so far, the average per completed iteration and
the estimated time left (testLoadTiming / testDurationText /
testIterAvgText), recomputed on every 2 s status poll.  A finished run
keeps the server's final elapsed_ms/avg_ms (no estimate), and before the
first completed iteration only the elapsed time is shown - no bogus
average or ETA.

The timing is client-side over the existing load block (started, done,
iterations), so it works with the running server as well; no server
change is needed.

Tests: the timing math and the two formatters, the script head render and
the suite summary render (3 new).  1061 Python + 1074 frontend.
sw cache simple-v480; CHANGELOG [3.38.0], help EN/RU,
docs/test-scripts.md and the AGENTS notes.

CHANGELOG.md

@@ -21,6 +21,12 @@ every change carries a descriptive commit message.
   a target); the text field stays authoritative, so a name path or a file the
   lists do not carry can still be typed, and the picker preselects the current
   path when it is known.
+- The **live load-test stats** now include the timing: while a script or suite
+  run repeats, the status line shows the **elapsed time**, the **average per
+  completed iteration** and the **estimated time left**, recomputed on every
+  status poll (`testLoadTiming`; the final report keeps the server's
+  `elapsed_ms`/`avg_ms`).  No completed iteration yet means no bogus
+  average/estimate - only the elapsed time.
 
 ### Changed