[snps-fricke]

Description

Follow-up to #352 with two independent changes:

1. Protect TimeConfiguration::_blocking with the mutex.
   Re-registering the async step handler from a user thread while the step is still open (the supported way to change the step size) races with ExecuteSimStep(), which reads the flag after the handler returned.
   See 367811e for a reproducer. It passes in a normal build and is reported by TSan without this fix.

2. Reject switching between SetSimulationStepHandler and SetSimulationStepHandlerAsync once the participant is Running (SilKit::StateError).
   Switching before Running and re-registering a handler of the same kind at any time (e.g. to change the step size) is still allowed.

   Why: the mutex from 1. does not make the switch safe, a single step still reads the flag several times (IsBlocking(), IsSimStepSync()) and can see different values.

   • Most significant: a sync handler can run while the previous async step is still open. After the async handler returned and before CompleteSimulationStep(), a switch to sync makes the next NextSimTask take AdvanceTimeSimStepSync(), which, unlike AdvanceTimeSimStepAsync(), does not check _isExecutingSimStep.
     See temporary commit a908a6a for reproducers.
   • Otherwise non-functional issues: wrong step metrics in ExecuteSimStep(), stale EvaluateAggregationInfo() results.

   Rejecting this technically breaks compatibility, though I think it is unlikely that users switch between sync/async handlers at runtime. However, since re-registration is the advertised API for changing the step size, this became somewhat more prominent.

I think 1. is straightforward, 2. may need discussion. An alternative to rejecting may be to guard AdvanceTimeSimStepSync() with _isExecutingSimStep and fix metrics/aggregation separately.

Developer checklist (address before review)

• Changelog.md updated
• Prepared update for depending repositories
• Documentation updated (public API changes only)
• API docstrings updated (public API changes only)
• Rebase → commit history clean
• Squash and merge → proper PR title