[mbrunnen]

Fixes #27.

validateConnectorRelatedFeatures now reports a concrete connector with no
ends, and validateBindingConnectorIsBinary a binding connector with no
ends, abstract or not
(KerML 1.0, 8.3.4.5.3,
8.3.4.5.2). A message
is lowered with isAbstract: true
(SysML 2.0, 8.2.2.16,
7.16.2). Both
belong together: a message has no ends, so the wider rule alone would flag
every message.

How to test

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

Look closely at

• The corpus findings do not change. Without the isAbstract fix, 8
  messages in the training material, Annex A and the interaction examples
  would be flagged.
• The full-form baseline moves only in the FlowUsage isAbstract row: 40 of
  the 138 corpus FlowUsages are messages.
• The opensysml fixture
  negative/semantic/k49-binding-connector-with-three-ends.kerml gains a
  finding on its abstract binding b2;, which the fixture marks as invalid.
• tests/check.rs used connection x : I; as a typing shorthand. It is
  abstract connection x : I; now, so the test keeps its intent: no finding
  at all.
• Associations (assoc R;) are left out, as the issue explains.
• The CHANGELOG.md entry is under a new Unreleased heading. Move or drop
  it as you prefer.