add drawable ellipse support

[elcritch]

Summary

• add first-class axis-aligned ellipse drawable operations
• render ellipse fills and strokes through the OpenGL, Metal, and Vulkan SDF paths
• provide an adaptive legacy texture-path fallback
• document the public API and cover construction, transfer, dispatch, and degenerate radii

Validation

• nim test (32 test files)
• GLSL fragment shader validation
• Vulkan SPIR-V validation and regeneration
• native ABI generation