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/index.html

@@ -1245,7 +1245,8 @@
 					<div id="esim-notifications" class="text-xs"></div>
 				</fieldset>
 			</div>
-			<div id="phone-sub-test" class="hidden">
+		</div>
+		<div id="phone-sub-test" class="hidden">
 				<div id="test-run-banner" class="hidden mb-3 px-3 py-2 rounded border border-amber-400 dark:border-amber-600 bg-amber-50 dark:bg-amber-900/20 text-sm text-amber-700 dark:text-amber-300"></div>
 				<div class="flex flex-wrap items-end gap-2 mb-3">
 					<div>
@@ -1664,7 +1665,7 @@
 // ===== Version =====
 // Single source of truth for the PWA version: shown in the header and used
 // by the server version check in pysimConnect().
-const SIMPLE_VERSION = '3.6.0';
+const SIMPLE_VERSION = '3.6.1';
 document.getElementById('app-version').textContent = 'v' + SIMPLE_VERSION;
 
 // ===== Tab switching =====