Add visual repro for incorrect bottom courtyard color

[anil08607]

Summary

Adds a passing visual repro for the PCB bottom-courtyard color issue.

The test renders courtyard primitives directly with altiumts:

• Mechanical 15 represents the top courtyard.
• Mechanical 16 represents the bottom courtyard.
• Tracks and a region cover Mechanical 15.
• A full-circle arc covers Mechanical 16.

Current behavior

Mechanical 16 incorrectly uses the same magenta color (#ec4899) as Mechanical 15.

The test explicitly confirms that both layers receive the same color and includes a single Altium SVG snapshot demonstrating the issue.

Notes

This PR only adds the reproduction. It does not fix the layer-color mapping and does not use Circuit JSON.

Test

bun test tests/svg/pcb-courtyards.test.ts

[vercel]

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