feat: Add radial / hub-and-spoke Conceptual DSL diagram type

[Copilot]

Summary

Implements the diagram: radial Conceptual DSL type as specified in the issue. Hub-and-spoke diagrams are common in strategy and architecture presentations — a central concept surrounded by supporting pillars, capabilities, or themes.

Syntax

diagram: radial
center: Platform
items:
  - Security
  - Reliability
  - Developer Experience
  - Observability

Changes

Parser (ConceptualDslParser.Radial.cs)

• Parses center: as the hub label and items: as the spoke list
• Validates 3–8 items (error otherwise); requires non-empty center:
• Creates center node (center) with radial:isCenter = true metadata
• Creates item nodes (item_0…item_N-1) with radial:itemIndex metadata
• Adds EdgeRouting.Straight spoke edges from center to each item

Layout (DefaultLayoutEngine.Radial.cs)

• Center node rendered as Shape.Circle, sized to fit its label
• Item nodes placed evenly on a computed radius starting at 12 o'clock (−π/2)
• Radius accounts for both item node spacing and minimum gap from center
• Colors: center node gets palette[0]; items cycle through remaining palette entries
• Title offset respected when diagram.Title is set

Tests

• Parser tests (15 new): CanParse, nodes/labels, metadata keys, spoke edge creation, straight routing, item count validation (2/9 → error; 3/5/8 → ok), missing section errors, empty center error, CR/LF line endings
• Layout tests (6 new): center is Circle shape, items equidistant from hub, hub at geometric center of items, first item at top, all nodes positive size, all positions non-negative
• E2E snapshot: conceptual-radial.input + conceptual-radial.expected.svg

Docs

• README.md: gallery thumbnail, chooser table row, #### radial syntax section
• doc/PackageReadme.md: radial added to supported diagram list

Test results

All 558 tests pass (497 unit + 61 E2E snapshots).

Security

No vulnerabilities found (CodeQL: 0 alerts).

[Copilot]

Pull request overview

Adds a new Conceptual DSL diagram type (diagram: radial) to DiagramForge, enabling hub-and-spoke (radial) diagrams end-to-end through parsing, layout, docs, and snapshot coverage.

Changes:

• Introduces radial parsing (center: + items:) with validation and straight-routed spoke edges.
• Implements a radial layout that positions items evenly around a circular center node with theme palette coloring.
• Adds unit tests, E2E fixtures/snapshot, and documentation updates for the new diagram type.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/DiagramForge/Parsers/Conceptual/ConceptualDslParser.cs	Updates supported-diagram-type documentation to include radial.
src/DiagramForge/Parsers/Conceptual/ConceptualDslParser.Dispatch.cs	Registers the radial parser handler.
src/DiagramForge/Parsers/Conceptual/ConceptualDslParser.Radial.cs	Implements radial DSL parsing (center/items, validation, nodes, straight edges, metadata).
src/DiagramForge/Layout/DefaultLayoutEngine.Conceptual.cs	Registers the radial conceptual layout handler.
src/DiagramForge/Layout/DefaultLayoutEngine.Radial.cs	Implements radial node sizing/placement and palette-based styling.
tests/DiagramForge.Tests/Parsers/ConceptualDslParserTests.cs	Adds parser test coverage for radial parsing, metadata, routing, and validations.
tests/DiagramForge.Tests/Layout/DefaultLayoutEngineTests.cs	Adds layout tests for radial geometry and basic invariants.
tests/DiagramForge.E2ETests/Fixtures/conceptual-radial.input	Adds an E2E fixture input for the new radial diagram.
tests/DiagramForge.E2ETests/Fixtures/conceptual-radial.expected.svg	Adds the expected SVG snapshot output for radial rendering.
doc/PackageReadme.md	Updates supported-diagram list to include conceptual funnel + radial.
README.md	Adds radial to the gallery and documents syntax/usage in the Conceptual DSL section.

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[jongalloway]

@copilot address comments

[Copilot]

@copilot address comments

Fixed in fa1bb13. Items now cycle over palette[1..] when the palette has more than one entry, so they can never wrap back to palette[0] (the center color). The same slicing logic is applied to NodeStrokePalette. For a single-color palette, all nodes share that one color as before.