v3.22.4: the proactive-drain pending exemption

The runner's unexpected-pending-command guard fired for any non-expect step,
so a proactive-drain after a step that answered 91XX failed with
"unexpected proactive command pending (SW 91XX) - add an expect step or a
status action" - contradicting the drain's own doc ("consume whatever the
card announces") and the v3.22.2 RAM recipe ("an expect or a proactive-drain").

The guard now exempts a proactive-drain step: its STATUS poll re-announces
the unfetched command, fetches and answers it, and the drain's final-SW and
pending handling is unchanged (a still-pending card is handed to the next
step, an idle card passes).  No pending-length plumbing: STATUS-first keeps
the drain's attempts/log semantics and tolerates a card that dropped the
command.  The guard note now names the drain instead of the misleading
"status action" (a status action in that position also trips the guard).

Docs: the drain section in test-scripts.md, api.md's action list and the
help EN/RU drain bullet.  Tests: 839 frontend + 819 Python; sw cache
simple-v408.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v407';
+const CACHE = 'simple-v408';
 const URLS = [
   'index.html',
   'help.html',