AddTransition's graph walk kept its visited, leaf and ignore sets in static
lists, so two Minions built at once on different threads (zones are built in
parallel) walked and cleared each other's state. Soul hit it as "collection
was modified" during parallel test runs. The sets are now the call's own.
BaseStateTests starts sixteen walks together behind a barrier, fifty rounds,
over prebuilt 200-state chains; with the statics restored every run failed
(several hundred of 800 walks wrong or throwing); with locals, clean.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
