feat: add vegetation index module to kornia-imgproc (ExG)

🚀 Feature Description

Add a vegetation index module to kornia-imgproc, starting with the Excess Green Index (ExG) — a widely used primitive for agricultural computer vision applications.

📂 Feature Category

Rust Core Library

💡 Motivation

Vegetation indices are fundamental for agricultural CV pipelines — crop monitoring, disease detection, and autonomous drone spraying. Having ExG as a native kornia-imgproc primitive benefits anyone building precision agriculture applications on top of kornia-rs.

💭 Proposed Solution

New module: kornia-imgproc/src/vegetation/ Function: excess_green(src: &Image<f32,3>, dst: &mut Image<f32,1>) Formula: ExG = 2*G - R - B, clamped to [0.0, 1.0] Follows existing kornia-imgproc API patterns (same signature as gray_from_rgb). Implementation already available in PR #833.

📚 Library Reference

Excess Green Index (ExG): Woebbecke et al. (1995) "Color indices for weed identification under various soil, residue, and lighting conditions"

🔄 Alternatives Considered

No response

🎯 Use Cases

• Autonomous drone spraying: identify vegetation in real time
• Crop health monitoring: detect stress and disease regions
• Wildfire detection: isolate green vegetation from burned areas
• Precision agriculture pipelines on edge hardware (Jetson Orin)

📝 Additional Context

No response

🤝 Contribution Intent

• I plan to submit a PR to implement this feature
• I'm requesting this feature but not planning to implement it