test runner: add the stuck-card watchdog

A card that stops answering (no SW, no driver error) leaves the runner
inside a blocking SCardTransmit while it holds the card lock - Stop
cannot take effect, the presence monitor waits for the lock and a load
test hangs in that iteration without explanation.

The runner now beats a progress heartbeat (each step start/finish and
each attempt of a bounded STATUS/drain loop); after _TEST_STEP_TIMEOUT
(30 s) the watchdog flags the run `stuck`, records the step and the idle
time, sets the stop flag and tries to cancel the blocked transmit by
disconnecting the PC/SC link on a daemon thread.  The disconnect must
run in the background: pcsc-lite serializes it behind the pending
transmit (both take the per-context lock), so a synchronous call would
block the watchdog instead of cancelling anything.  Where a platform
does cancel, the step fails through the ordinary on_fail path; either
way the finalisers map `stuck` to `error` - a stuck card is not a user
stop - and the pending drain is skipped on the dead link.  The
transport is marked stale once the disconnect returns, so the next equip
rebuilds it.

The run status gains `stuck` and `heartbeat`.  Help EN/RU,
docs/test-scripts.md, docs/api.md, the CHANGELOG (folded into the
unreleased 3.37.0) and both AGENTS describe the behaviour and the
background-disconnect caveat; the sw cache is simple-v476 (help is
precached).

Tests: tests/test_test_watchdog.py (17: the check rule, the abort, the
final status of the single/load/suite runs and a blocked-runner
integration case).  1058 Python + 1069 frontend.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v475';
+const CACHE = 'simple-v476';
 const URLS = [
   'index.html',
   'help.html',