[mbrunnen]

Fixes #23.

validateStateDefinitionParallelSubactions and
validateStateUsageParallelSubactions now report a parallel state that owns
a transition, and no longer a state that owns a plain succession. SysML 2.0
forbids only transitions there
(8.3.18.5,
8.3.18.6).

How to test

$ cargo test -p sysmlv2-model --lib a_parallel_state_may_own_successions
$ cargo test -p sysmlv2-parser --test integration opensysml::

Look closely at

• This departs from the Pilot Implementation, which also rejects a
  succession. The opensysml fixtures
  negative/semantic/s33-parallel-state-usage-with-succession.sysml and
  negative/xpect/p19-parallel-state-with-transition.sysml come from its
  reject tests and hold a succession, not a transition. The fixtures are
  unchanged; both baselines record them as unknown now. The probes with a
  transition keep their error.
• The message changes to "A parallel state cannot own transitions".
• The CHANGELOG.md entry is under a new Unreleased heading. Move or drop
  it as you prefer.