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.

docs/test-scripts.md

@@ -242,7 +242,10 @@ between iterations is an error.  The SCP80 counter state is shared across
 iterations, so packets continue from what the previous iteration consumed.
 The iteration count is a run parameter - the stored script/suite is unchanged.
 The accepted range is **1 to 1,000,000,000** (the PWA clamps, the server
-answers 400 above it).
+answers 400 above it).  While the run repeats, the status line shows the
+elapsed time, the average per completed iteration and the estimated time
+left, recomputed on every poll; the final report keeps the server's
+`elapsed_ms`/`avg_ms`.
 
 ## A card that stops answering