Add Phase Vortex quantum brush

[qdwg]

Summary

• Adds a new phasevortex brush for UnitaryHACK issue Make your own Quantum Brush! #50.
• Maps stroke geometry into a Mach-Zehnder-style interference phase field.
• Includes brush metadata, parameter controls, README explanation, and generated before/after example images.

Validation

• Ran phasevortex.run() directly on a synthetic RGBA canvas with a spiral stroke.
• Confirmed output shape and dtype match the input image.
• Confirmed 47,756 pixels changed in the generated example image.
• Ran python -m py_compile effect/phasevortex/phasevortex.py.
• Validated effect/phasevortex/phasevortex_requirements.json as JSON.

AI disclosure

This contribution was developed with assistance from OpenAI Codex. I reviewed and tested the resulting code before submitting.

Addresses #50.