Render class nodes with compartments and UML relationship styles

[Copilot]

Teaches the SVG renderer to draw UML-style class boxes and the core relationship line/arrow variants required by Mermaid class diagrams. All changes are renderer + layout only — no parser dependency.

Model

• Edge.SourceArrowHead (ArrowHeadStyle, default None) — source-end marker for composition/aggregation diamonds, symmetric with the existing ArrowHead (target end).

Layout

• SizeClassNode — computes node dimensions from annotation lines, class name, and per-compartment line counts. Called instead of SizeStandardNode when Node.Compartments.Count > 0 || Node.Annotations.Count > 0.
• AnnotationFontSizeRatio = 0.85 extracted as a named constant (shared between layout and renderer).

Rendering

New SVG markers in SvgRenderSupport.AppendDefs:

Marker ID	Shape	Used for
arrowhead-open	hollow triangle	inheritance, realization
arrowhead-filled-diamond	filled diamond	composition (source end)
arrowhead-open-diamond	hollow diamond	aggregation (source end)

SvgStructureWriter.AppendEdge now emits the correct marker-end / marker-start based on ArrowHead and SourceArrowHead values.

SvgNodeWriter.AppendClassNode — new method rendering UML class boxes:

• rx=0 outer rect (sharp UML corners)
• Italic «stereotype» annotations above the class name
• Bold centered class name
• Horizontal <line> dividers above each compartment
• Left-aligned compartment content (attributes, methods, etc.)

var node = new Node("Animal", "Animal");
node.Annotations.Add(new Label("abstract"));
node.Compartments.Add(new NodeCompartment("attributes", [new Label("-name: String")]));
node.Compartments.Add(new NodeCompartment("methods", [new Label("+makeSound(): void")]));

var inheritance = new Edge("Dog", "Animal") { ArrowHead = ArrowHeadStyle.OpenArrow };
var composition = new Edge("Dog", "Tail") { SourceArrowHead = ArrowHeadStyle.Diamond, ArrowHead = ArrowHeadStyle.None };
var aggregation = new Edge("Pack", "Dog")  { SourceArrowHead = ArrowHeadStyle.Circle,  ArrowHead = ArrowHeadStyle.None };

Tests

• 26 new rendering tests in SvgClassDiagramRendererTests — compartment structure, stereotype rendering, sizing, and all 8 UML relationship types (inheritance, realization, composition, aggregation, association, link, dependency, dashed link).
• Edge_SourceArrowHead_DefaultsToNone added to model tests.
• All 61 E2E snapshot fixtures updated — only the <defs> section changes (three new marker definitions); no existing node/edge output affected.

[Copilot]

Pull request overview

This PR adds UML class diagram rendering capabilities to DiagramForge's SVG renderer, including compartmented class boxes and UML relationship arrow/marker variants. All changes are renderer and layout only — no parser dependency.

Changes:

• Adds Edge.SourceArrowHead property and three new SVG marker definitions (arrowhead-open, arrowhead-filled-diamond, arrowhead-open-diamond) for UML relationships.
• Implements SizeClassNode layout sizing and AppendClassNode rendering for UML-style class boxes with annotations, bold class names, divider lines, and left-aligned compartments.
• Adds 26 new unit tests for class node rendering and relationship styles, plus a model test, and updates all 61 E2E snapshot fixtures with the new <defs> markers.

Reviewed changes

Copilot reviewed 7 out of 68 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/DiagramForge/Models/Edge.cs	Adds SourceArrowHead property for source-end markers
src/DiagramForge/Layout/DefaultLayoutEngine.cs	Adds SizeClassNode method and AnnotationFontSizeRatio constant; routes class nodes to new sizing logic
src/DiagramForge/Rendering/SvgRenderSupport.cs	Adds three new SVG marker definitions in AppendDefs
src/DiagramForge/Rendering/SvgStructureWriter.cs	Expands AppendEdge to emit marker-start/marker-end based on SourceArrowHead/ArrowHead values
src/DiagramForge/Rendering/SvgNodeWriter.cs	Adds AppendClassNode method and AnnotationFontSizeRatio constant; branches to class rendering when compartments/annotations are present; suppresses standard label for class nodes
tests/.../SvgClassDiagramRendererTests.cs	New test file with 26 tests for class nodes and UML relationship rendering
tests/.../DiagramModelTests.cs	Adds Edge_SourceArrowHead_DefaultsToNone test
tests/.../Fixtures/*.expected.svg (61 files)	Updates <defs> with new markers; minor whitespace change in <path> elements

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[jongalloway]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Both fixes applied in a6e6215:

1. Annotation width — SizeClassNode now estimates width as «{ann.Text}» (matching the guillemet wrapping in AppendClassNode) so the node is never narrower than the rendered stereotype text.

2. markerStart completeness — added Arrow and OpenArrow cases to mirror the markerEnd switch, so all four ArrowHeadStyle values are handled symmetrically at both ends.