feat(features2d): implement BFMatcher, DMatch and drawing utilities (#75)

[kalwalt]

PR Summary

• PR Type: Feature (feat)
• Impact: 7 files changed, 1029 insertions.
• Review Time: ~10-15 minutes.
• Milestone: Feature Matching Infrastructure
• Issue Link: Resolves Implement BFMatcher and DMatch for feature correspondence #75

Detailed Description

This PR implements the core brute-force descriptor matching and visual correspondence drawing infrastructure for purecv in the new modules:

1. src/features2d/matcher.rs:
   • DMatch structure for match representation matching OpenCV fields (query_idx, train_idx, train_img_idx, distance).
   • DescriptorMatcher<T> trait to define standard matcher behavior.
   • BFMatcher<T> supporting NormHamming (binary like ORB) and NormL2 (float).
   • Support for cross-check mutual consistency matching.
   • Support for parallel matching using rayon under the parallel feature gate.
   • filter_matches helper implementing Lowe's ratio test.
2. src/features2d/draw.rs:
   • draw_keypoints and draw_matches functions written in pure Rust (100% WebAssembly compatible, no external graphics dependencies).
   • Integer-only Bresenham's line and Midpoint circle drawing algorithms.
   • Support for pseudo-random line/circle colors and optional hiding of unmatched keypoints.
3. examples/match_features.rs:
   • An example loading, splitting, and matching ORB features on the stitched graf.png image.
   • Saves the visual output showing multi-colored match lines over a grayscale background.

Review Checklist

• Code follows project style guidelines.
• LGPL license headers are present on all new .rs files.
• All functions are generic, safe, and return Result<T, PureCvError>.
• No unsafe blocks are introduced.
• Rayon parallelism is feature-gated under #[cfg(feature = "parallel")].
• Standard formatting (cargo fmt) and linting (cargo clippy) pass with zero warnings.

Risk Assessment

• Risk Level: Low.
• Size: Moderate (~1000 LOC, including extensive test coverage and visual example).
• Impact: Adds isolated new matching modules without touching existing core/imgproc logic.
• Mitigation: All new code has 100% unit test coverage.

Test Coverage

• Coverage: Implemented comprehensive tests in src/features2d/tests.rs for:
  • Hamming & L2 metrics validation.
  • KNN matching and Lowe's ratio filtering.
  • Cross-check mutual matching.
  • Coordinate clipping and primitive circle/line drawing boundaries.
• Status: 318/318 tests successfully passing.

Visual Aids

Here is the output from cargo run --example match_features:

Review Automation

• All pre-commit check scripts (formatting, clippy, unit tests) completed successfully.