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.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.6.0"
+version = "3.6.1"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 requires-python = ">=3.8"
 # pysim is a git-only dependency installed explicitly by setup.bat/setup.sh.