v3.37.0: load testing for test scripts and suites

POST /api/test/run takes an optional iterations count (the PWA's
Iterations field): the run repeats, successful iterations are discarded
(steps/log cleared, only load.done and the timing grow), the first error
stops the loop and keeps that iteration's report, a stop ends it after
the current iteration, and a card session change between iterations is
an error.  The SCP80 counter state is shared across iterations, so
packets continue from what the previous run consumed.

The status gains a load block (iteration/done/failed_iteration/
elapsed_ms/avg_ms; disabled for a normal run).  The suite runner was
split into a per-iteration body (_test_suite_execute_once) + the
single-run finaliser, and two load workers own the loops.

Review fixes: a raised runner error finalises as error (no stuck
running state), a suite-internal session change is an error, a stopped
member ends the suite loop, the final status keeps a warning from any
iteration, avg_ms covers the attempted iterations, and each suite
iteration's counters_before is read fresh.

1041 Python + 1069 frontend; simple-v475.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.36.0"
+version = "3.37.0"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 license = { text = "GPL-2.0-or-later" }
 requires-python = ">=3.9"