perf(vision): accelerate exact distance transform

[rsasaki0109]

Summary

• accelerate exact unit-spacing EDT with compact binary-row distances, tiled transposes, and bounded Rayon dispatch
• add explicit Rust/Python output and workspace reuse, including packed 0/255 input without a normalization copy
• extend the OpenCV harness with allocate/reuse EDT rows and refresh README/Pages/roadmap receipts

Performance

• native Criterion 4K allocate: ~451.63 ms before, ~75 ms after
• native Criterion 4K reusable output/workspace: ~43 ms
• Python/OpenCV 4K allocate gap: OpenCV 12.35x before, 1.63x after
• Python/OpenCV 4K reuse gap: OpenCV 1.07x on the final interleaved run
• canonical OpenCV accuracy: exact fraction 1.0 and max error 0.0 at VGA/1080p/4K

Machine-specific details and limitations are in notes/2026-07-15_exact_edt_acceleration.md.

Validation

• cargo test -p spatialrust-vision --features full --lib (96 passed)
• EDT full-feature property test (passed)
• cargo clippy -p spatialrust-vision --features dense --all-targets -- -D warnings
• focused Python binding tests and mypy.stubtest (passed)
• OpenCV performance report contract (8 passed)
• workspace rustdoc for spatialrust-vision (passed)
• final seeded/interleaved OpenCV performance suite at VGA/1080p/4K (passed)

The broader full-feature Clippy command still reaches the existing spatialrust-camera/src/rgbd.rs manual div_ceil warning; this focused PR does not modify that unrelated baseline issue.