load tests: show the live timing in the run banner

The test-run banner (visible on every Test-script view) now carries the
same live stats as the script head and the suite summary: elapsed so far,
the average per completed iteration and the estimated time left.  The
text is built by a pure helper (testRunBannerText) so it is testable;
testRender only calls it while a run is active, and non-load runs keep
their position text (step or suite-member progress).

The existing banner-source assertion moved to the helper.  Tests: the
banner text for a load suite and a plain run.  1061 Python + 1075
frontend; sw cache simple-v481.  CHANGELOG/help/docs/AGENTS updated.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v480';
+const CACHE = 'simple-v481';
 const URLS = [
   'index.html',
   'help.html',