Add the QGraphicsView layer: NodeItem (SVG symbol + title + draggable,
snap-to-grid), PortItem (relation-colored anchors with compatibility
hints), EdgeItem (orthogonal routing, arc hops on crossings, line styles,
head/tail decorations, animated flow dashes), and DiagramScene binding the
document to items with edge-drawing interaction and a grid background.
Add EditorController with a QUndoStack (add/connect/delete/move/edit) and
scene interaction tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
