Tighten API naming, close NodeRole enum gap, protect newNodeId

[imakris]

Summary

Addresses the last 3 remaining API issues from the thorough code review, plus a docs formatting bug:

• Rename getNodeId/getPortIndex → connectionNodeId/connectionPortIndex — the old names read as "get a node's ID" when they actually extract a field from a ConnectionId for a given port side. The new names make the intent clear.
• Close NodeRole enum gap — OutPortCount was 9 (skipping 8), causing all subsequent values to be non-sequential. Renumbered to OutPortCount=8, Widget=9, ValidationState=10, ProcessingStatus=11.
• Move newNodeId() from public to protected on AbstractGraphModel — this is a side-effectful ID generator that only subclasses and internal library code should call. External code should use addNode() instead. Internal callers (BasicGraphicsScene, PasteCommand) are added as friends.
• Fix docs table formatting in graph-models.rst — a .. note:: block was inserted mid-table, breaking the Caption and CaptionVisible rows. Moved the note after the complete table.

Test plan

• Verify library compiles with Qt5 and Qt6
• Run existing test suite (TestConnectionId, TestAbstractGraphModel, TestDataFlow, etc.)
• Verify examples build and run (enum renumbering is source-compatible since values are used symbolically, not as magic numbers)
• Check docs render correctly with the table fix

https://claude.ai/code/session_01BSbxaZ4ghZbro1BibMXJBY