Fix grayscale shading gradient for 2D angle coloring

[kryptokommunist]

Summary

• derive grayscale shading from each group's arc orientation so angles vary without relying on pattern settings
• normalize angle-to-intensity mapping to span black-to-white for visible gradients in the 2D view

Testing

• not run (not requested)

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Codex Task

[gemini-code-assist]

Summary of Changes

Hello @kryptokommunist, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new visualization capability to the Doyle Spiral Studio, allowing users to apply a grayscale shading gradient to 2D regions based on the orientation of their constituent arcs. This feature provides a distinct visual cue for angular variations, offering an alternative to pattern-based fills and improving the interpretability of the generated spirals.

Highlights

• New Feature: Grayscale Angle Shading: Introduced an option to color 2D regions based on their line angle using a grayscale gradient, providing a new visual representation.
• User Interface Integration: A new checkbox labeled 'Colour 2D regions by line angle (grayscale)' has been added to the advanced settings in the UI, allowing users to toggle this feature.
• Core Logic Implementation: The doyle_spiral_engine.js file was updated to calculate the normalized angle for each arc group and map it to a grayscale intensity, which is then applied as a solid fill.
• Solid Fill Support: The ArcGroup class has been enhanced to support solid color fills, a prerequisite for applying the new grayscale shading mechanism.

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[gemini-code-assist]

Code Review

This pull request introduces a new feature to color 2D regions by their line angle in grayscale, which is a great addition for visualizing the spiral's structure. The implementation is solid: a new shadeByAngle option is added and plumbed through the application, and the rendering logic correctly calculates grayscale values based on arc orientation. The code is clear and the changes are well-contained. I have one suggestion to improve code clarity by removing a magic number in the intensity calculation.

[gemini-code-assist]

Using 180 instead of 179.999 for normalization would be clearer and avoid a magic number. Since normalizedAngle is always in the range [0, 180), dividing by 180 will produce a value in [0, 1). When multiplied by 255 and rounded, this will correctly map to the [0, 255] integer range for the color channel, effectively covering the full black-to-white spectrum as intended.

Suggested change

	const intensity = clamp(normalizedAngle / 179.999, 0, 1);
	const intensity = clamp(normalizedAngle / 180, 0, 1);