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.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v479';
+const CACHE = 'simple-v480';
 const URLS = [
   'index.html',
   'help.html',