perf(vision): fuse paired Sobel gradients

[rsasaki0109]

Summary

• add exact OpenCV-compatible paired 3x3 Sobel APIs with caller-owned output reuse
• add a fused abs(Gx) + abs(Gy) path that avoids materializing two gradient images
• route the Canny 3x3 aperture through the paired gradient implementation
• expose paired and fused operations in Python with shape, contiguity, aliasing, and reuse validation
• add Criterion and OpenCV comparison harnesses plus GitHub Pages algorithm documentation

Why

The previous generic Sobel path traversed the source independently for X and Y and allocated intermediate images. This slice establishes a specialized paired contract and a canonical fused workload where SpatialRust can outperform OpenCV without relaxing result accuracy.

Measured impact

Windows 11, Intel 6C/12T, OpenCV 4.13.0, OpenCL disabled, 300 randomized exactness cases:

Resolution	OpenCV allocated pipeline	SpatialRust fused	Result
1080p	5.508 ms	2.954 ms	SpatialRust 1.86×
4K	22.502 ms	10.262 ms	SpatialRust 2.19×
8K	97.258 ms	40.238 ms	SpatialRust 2.42×

The claim is intentionally scoped to allocated L1 magnitude. Caller-owned reuse is approximately tied at 1080p and OpenCV remains slightly faster at 4K/8K. Standalone paired gradients also remain an optimization target.

Validation

• cargo test -p spatialrust-vision --features full — 113 unit + 11 property tests passed
• cargo clippy -p spatialrust-vision --features imgproc-canny -- -D warnings
• cargo test -p spatialrust-vision --features imgproc-canny --lib
• cargo test -p spatialrust-vision --features imgproc-filter advanced_filter --lib
• cargo bench -p spatialrust-vision --features imgproc-filter --bench filter --no-run
• cargo doc -p spatialrust-vision --features imgproc-canny --no-deps
• Python binding suite passed (one existing skip)
• OpenCV conformance: zero mismatch for Sobel and Canny apertures 3/5/7, L1/L2
• focused OpenCV harness: 300 randomized exact cases passed

Rust 1.82 dependency resolution is currently blocked before compilation by the repository's floating clap_builder 4.6.0 requiring Edition 2024; this is independent of the Sobel implementation.