feat(features2d): add module skeleton and public API

[kalwalt]

feat(features2d): Add module skeleton and public API (#56)

1. PR Summary

This PR introduces the initial module skeleton and public API surface for a new features2d module modeled after OpenCV's API.

• Impact: 5 files changed (+504 lines, 0 deletions)
• Risk Level: 🟢 Low (Skeleton and API surface setup only, no algorithm logic implemented yet)
• Review Time: ~5–10 minutes
• Target Branch: dev

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2. Detailed Description

This pull request fulfills Issue #56 by adding the foundational directory, files, structs, enums, configurations, and public exports to support future FAST keypoint detection and ORB descriptor extraction.

Core Changes:

• 📁 src/features2d/: Created the new module directory.
• 🔧 src/features2d/mod.rs: Publicly exports:
  • KeyPoint
  • FastFeatureDetector, FastType
  • Orb
• 🔧 src/features2d/keypoint.rs: Implements the KeyPoint structure mirroring cv::KeyPoint with standard properties, custom constructors, and default parameter initialization.
• 🔧 src/features2d/fast.rs: Sets up FastFeatureDetector and FastType with standard getters/setters, default configurations matching OpenCV defaults, and signature placeholder for .detect().
• 🔧 src/features2d/orb.rs: Sets up Orb and ScoreType with full customizable getters/setters matching OpenCV parameter controls, default builders, and signature placeholders for .detect(), .compute(), and .detect_and_compute().
• 🏗️ src/lib.rs: Declared and re-exported pub mod features2d;.

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3. Review Checklist

General

• Code follows project style guidelines.
• Self-review completed.
• Comments added for complex logic.
• No sensitive data exposed.

Code Quality

• Functions are focused, clear, and small.
• Variable names are descriptive and match OpenCV parity.
• Standard Default and constructor implementations are introduced.

Documentation

• Comprehensive doc comments (///) added referencing official OpenCV main documentation links for every type.

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4. Risk Assessment

• Overall Risk Score: 2.0 / 10 (Low Risk)
• API Compatibility: Fully backwards compatible; only exposes new public types and modules without affecting existing core or imgproc code.
• Mitigation: Verified skeleton structure compiles successfully with zero warnings/errors.

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5. Test Coverage

Metric	Base Branch (dev)	This Branch	Change
Compile Errors	0	0	None
Clippy Lints	0	0	None
Unit Tests Passed	259 / 259	259 / 259	No regressions
Doc Tests Passed	28 / 28	28 / 28	No regressions

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6. Architecture Diagram

graph TD
    purecv[purecv crate] --> core[core module]
    purecv --> imgproc[imgproc module]
    purecv --> video[video module]
    purecv --> features2d[features2d module]
    
    subgraph features2d
        mod.rs[mod.rs exports] --> keypoint[keypoint::KeyPoint]
        mod.rs --> fast[fast::FastFeatureDetector]
        mod.rs --> orb[orb::Orb]
    end

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7. Review Automation

• Ran cargo fmt to verify and align all file styling.
• Ran cargo clippy to check for lints, with zero issues detected.
• Ran cargo test to execute all unit tests, all of which pass cleanly.