feat(imgproc): implement corner detection pipeline — cornerSubPix, goodFeaturesToTrack, Harris, Shi-Tomasi, preCornerDetect

[Copilot]

What was implemented

New module: src/imgproc/feature.rs

Six new corner detection functions (all OpenCV-compatible, single-channel Matrix<T>, returning Result<…>):

Function	Description
corner_harris	Harris corner response map — det(M) − k·tr(M)²
corner_min_eigen_val	Shi-Tomasi response — minimum eigenvalue of the structure tensor
corner_eigen_vals_and_vecs	Full 6-channel structure tensor decomposition (λ1, λ2, x1, y1, x2, y2)
good_features_to_track	Filtered, NMS corner list with quality threshold + min-distance enforcement; supports both Harris and Shi-Tomasi scoring
corner_sub_pix	Iterative sub-pixel refinement via the Forstner/Lucas gradient method
pre_corner_detect	Ix²·Iyy + Iy²·Ixx − 2·Ix·Iy·Ixy pre-corner feature map

All functions support the parallel and simd feature flags and use BorderTypes for border extrapolation.

New example: examples/corner_detection.rs

Single file that groups all six corner detectors against a real image (defaults to butterfly.jpg, accepts any image via CLI):

cargo run --example corner_detection
cargo run --example corner_detection -- examples/data/graf1.png

Steps demonstrated:

1. Harris response map (normalized, saved as PNG)
2. Shi-Tomasi (min eigenvalue) response map
3. good_features_to_track — Shi-Tomasi mode, top 200 corners
4. good_features_to_track — Harris mode, top 200 corners
5. corner_sub_pix — sub-pixel refinement with before/after Δ printed
6. pre_corner_detect — feature map saved as PNG
7. corner_eigen_vals_and_vecs — λ1 channel extracted and saved

Module wiring

• src/imgproc.rs — pub use feature::*; added
• src/lib.rs — TermCriteria, TermType, Point2f, Size2i added to prelude

Tests

224 comprehensive unit tests added across src/imgproc/tests.rs:

• correctness for all six functions
• error cases (multi-channel input, invalid params)
• round-trip and numerical checks for corner_sub_pix

Checklist

• src/imgproc/feature.rs — all six corner detection functions
• examples/corner_detection.rs — unified example for all detectors
• src/imgproc.rs / src/lib.rs — public API wired up
• src/imgproc/tests.rs — 224 tests pass
• cargo fmt && cargo clippy — zero warnings
• cargo run --example corner_detection — verified on butterfly.jpg
• cargo test — all tests pass

Closes #24

[kalwalt]

@copilot if it is ok, we can merge it.