fix: close the eSIM panel so the Test script pill shows its panel (v3.6.1)

#phone-sub-esim was missing its closing </div> before #phone-sub-test, so
the browser parsed the test panel (and everything after) as a child of the
eSIM panel.  Clicking Test script unhid the panel itself, but its hidden
ancestor kept it invisible - the pill has never shown a panel in a real
browser since it was added (v3.5.11).  The eSIM LPA pill conversely showed
the whole test editor below its own fields.

The old whole-file div-count test stayed green because the inline script's
template strings happen to contain one extra </div>; the real markup was
409 opens vs 408 closes.

- add the missing </div>.
- html.test.js: count divs on the markup only (scripts excluded) and add a
  structural walk (stray end tags, cross-nesting, unclosed elements, HTML
  optional end tags honoured) over index.html, help.html and help-ru.html,
  plus a Simulator-panel sibling-depth check.  Run this for every HTML edit.
- the walk immediately found the same class of bug in both help pages: six
  unclosed <section> chapters each; closed them.
- AGENTS: the HTML structure test is now part of the working conventions.

602 frontend / 482 Python green; version 3.6.1; sw simple-v274.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v273';
+const CACHE = 'simple-v274';
 const URLS = [
   'index.html',
   'help.html',