Merge pull request #28216 from happy-capybara-man:4.x

js: restore deep copy behavior for Mat.clone() #28216

### Problem
In OpenCV.js, `cv.Mat.clone()` may resolve to Embind `ClassHandle.clone()` (handle/shallow clone) instead of OpenCV deep copy.

### Changes
- modules/js/src/helpers.js: override `cv.Mat.prototype.clone` -> `mat_clone` after runtime init
- modules/js/test/test_mat.js: update/extend tests to validate deep copy semantics of `clone()`

Fixes  #27572
Related: PR #26643 (js_clone_fix), PR #27985 (documentation update)

### Verification
- Built OpenCV.js with Emscripten 2.0.10
- QUnit: bin/tests.html
- CoreMat: test_mat_creation (0 failures)
<img width="745" height="362" alt="clone_fix" src="https://github.com/user-attachments/assets/16399abf-b94c-4591-b4aa-6e0fbd6cf23b" />

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/js/src/helpers.js

@@ -410,3 +410,19 @@ if (
     if (cv.UMat) cv.UMat.prototype[Symbol.dispose] = cv.UMat.prototype.delete;
     // Add more as OpenCV gains new manual-cleanup classes
 }
+
+// Override Emscripten's shallow clone() with OpenCV's deep copy mat_clone()
+// This restores the expected behavior where clone() performs a deep copy.
+// Background: Emscripten 3.1.71+ added ClassHandle.clone() which only does shallow copy.
+// See: https://github.com/opencv/opencv/pull/26643
+// See: https://github.com/opencv/opencv/issues/27572
+var _opencv_onRuntimeInitialized_backup = Module['onRuntimeInitialized'];
+Module['onRuntimeInitialized'] = function() {
+    if (_opencv_onRuntimeInitialized_backup) {
+        _opencv_onRuntimeInitialized_backup();
+    }
+    if (typeof cv !== 'undefined' && cv.Mat &&
+        typeof cv.Mat.prototype.mat_clone === 'function') {
+        cv.Mat.prototype.clone = cv.Mat.prototype.mat_clone;
+    }
+};

modules/js/test/test_mat.js

@@ -173,7 +173,7 @@ QUnit.test('test_mat_creation', function(assert) {
     //   clone
     {
         let mat = cv.Mat.ones(5, 5, cv.CV_8UC1);
-        let mat2 = mat.mat_clone();
+        let mat2 = mat.clone();
 
         assert.equal(mat.channels, mat2.channels);
         assert.equal(mat.size().height, mat2.size().height);