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.

CHANGELOG.md

@@ -22,11 +22,12 @@ every change carries a descriptive commit message.
   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.
+  run repeats, the **run banner** (visible on every Test-script view), the
+  script editor's result head and the suite summary show the **elapsed time**,
+  the **average per completed iteration** and the **estimated time left**,
+  recomputed on every status poll (`testLoadTiming`/`testRunBannerText`; 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