Add SIFT feature detector TypeScript definitions and enable NONFREE build flag

[Copilot]

Adds TypeScript type definitions for SIFT (Scale-Invariant Feature Transform) feature detector. SIFT patent expired in 2020 but requires OpenCV built with OPENCV_ENABLE_NONFREE=ON.

Changes

Type Definitions (src/types/opencv/SIFT.ts)

• SIFT class extending Feature2D with 5 optional constructor parameters matching OpenCV 4.12 API
• Exported via _types.ts

Build Configuration (.github/workflows/build-opencv.yml)

• Added --cmake_option="-DOPENCV_ENABLE_NONFREE=ON" to enable SIFT in future opencv.js builds

Documentation (README.md)

• Feature detection section with SIFT usage example and memory management patterns

Tests (test/SIFT.test.ts)

• Type definition validation (runs now)
• Runtime functionality tests (skipped until opencv.js rebuilt with NONFREE enabled)

Usage

const sift = new cv.SIFT(
  500,   // nfeatures
  3,     // nOctaveLayers  
  0.04,  // contrastThreshold
  10,    // edgeThreshold
  1.6    // sigma
);

const keypoints = new cv.KeyPointVector();
const descriptors = new cv.Mat();
sift.detectAndCompute(gray, new cv.Mat(), keypoints, descriptors);

console.log(`Found ${keypoints.size()} keypoints with ${descriptors.cols}-dim descriptors`);

// Cleanup
keypoints.delete();
descriptors.delete();
sift.delete();

Note: Full SIFT functionality requires rebuilding opencv.js binary with updated workflow. TypeScript definitions work immediately.

Original prompt

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This section details on the original issue you should resolve

<issue_title>could including SIFT function</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes could including SIFT function #62

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