v3.19.0 (review fixes): expose the menu-select Match case choice; doc/comment fixes

Review of 7250dbd:
- the step editor's menu-select form gains a Match case select
  (Case-insensitive (default) / Case-sensitive); the step summary flags a
  case-sensitive step with (case), the saved params carry
  case_sensitive: true only when chosen, and the docs say the editor
  authors the flag.
- docs/api.md: fix the 'with an optionally' grammar.
- the Variant-2 menu test's comment claimed a contains match while it is an
  exact one; a real contains leg is added ('ыбор' -> item 128).
- timeless rationale for the case-insensitive default; the format note now
  dates the change.
sw cache simple-v389 (index.html/help changed).
Tests: 821 frontend + 726 Python.

pysim_simple_server/testscript.py

@@ -281,9 +281,9 @@ def _normalise_params(kind, p):
             if mode not in ('exact', 'contains'):
                 raise ScriptError('menu-select: mode must be exact or contains')
             out['mode'] = mode
-            # Menu labels are UI text: case-insensitive by default (no test
-            # scripts predate this choice); `case_sensitive: true` forces an
-            # exact-case match.
+            # Menu labels are UI text: a case slip should not fail a run, so
+            # the match is case-insensitive by default; `case_sensitive: true`
+            # forces an exact-case match.
             out['case_sensitive'] = bool(p.get('case_sensitive', False))
         if not out:
             raise ScriptError('menu-select: item_id or text is required')