features2d: add OpenCL acceleration for BFMatcher cross-check

BFMatcher::match() with crossCheck=true previously skipped the OCL
dispatch in knnMatchImpl entirely, falling back to CPU even when UMat
inputs and an OpenCL device were available. This adds
ocl_matchWithCrossCheck() for CV_32FC1 descriptors (e.g. SIFT, SURF):
both the forward and reverse nearest-neighbour passes run on the GPU
via the existing ocl_matchSingle() kernel, then the cross-check filter
runs on the CPU. Only two small index arrays (1×N ints) are downloaded
— the O(N²×D) distance work stays on the device.

The OCL dispatch in knnMatchImpl is also refactored to unify the
Mat/UMat train collection selection before branching on crossCheck.

On a NVIDIA RTX 3060 with SIFT descriptors the OCL path is 6–9×
faster than CPU at 2k–10k features per image. Binary descriptors
(ORB, BRIEF — CV_8U) are unaffected; the existing type guard in
ocl_matchSingle keeps them on the CPU path as before.

Also adds a correctness test (Features2d_BFMatcher_CrossCheck) and an
OCL perf test (BruteForceMatcherFixture/MatchCrossCheck).

modules/features2d/test/test_matchers_algorithmic.cpp

@@ -629,4 +629,47 @@ TEST(Features2d_DMatch, issue_17771)
     EXPECT_NO_THROW(ubf->knnMatch(usources, utargets, match, 1, mask, true));
 }
 
+// Verify that cross-check BFMatcher gives identical results via Mat (CPU) and UMat (OCL or CPU).
+// When OpenCL is active the UMat path exercises ocl_matchWithCrossCheck; when it is not,
+// both paths fall through to the same CPU code — either way the results must match.
+TEST(Features2d_BFMatcher_CrossCheck, ocl_matches_cpu)
+{
+    RNG rng(42);
+    const int nQuery = 200;
+    const int nTrain = 400;
+    const int dim    = 128;
+
+    // Float descriptors: the OCL dispatch in knnMatchImpl requires CV_32FC1
+    Mat queryMat(nQuery, dim, CV_32FC1);
+    Mat trainMat(nTrain, dim, CV_32FC1);
+    rng.fill(queryMat, RNG::UNIFORM, 0.f, 1.f);
+    rng.fill(trainMat, RNG::UNIFORM, 0.f, 1.f);
+
+    // CPU reference: Mat inputs always take the CPU path
+    Ptr<BFMatcher> cpuMatcher = BFMatcher::create(NORM_L2, true /*crossCheck*/);
+    vector<DMatch> cpuMatches;
+    cpuMatcher->match(queryMat, trainMat, cpuMatches);
+
+    // UMat path: activates OCL dispatch when OpenCL is available
+    UMat queryUMat = queryMat.getUMat(ACCESS_READ);
+    UMat trainUMat = trainMat.getUMat(ACCESS_READ);
+    Ptr<BFMatcher> oclMatcher = BFMatcher::create(NORM_L2, true /*crossCheck*/);
+    vector<DMatch> oclMatches;
+    oclMatcher->match(queryUMat, trainUMat, oclMatches);
+
+    // Both paths must return the same set of matches (order may differ)
+    ASSERT_EQ(cpuMatches.size(), oclMatches.size());
+
+    auto byQuery = [](const DMatch& a, const DMatch& b) { return a.queryIdx < b.queryIdx; };
+    sort(cpuMatches.begin(), cpuMatches.end(), byQuery);
+    sort(oclMatches.begin(), oclMatches.end(), byQuery);
+
+    for (size_t i = 0; i < cpuMatches.size(); ++i)
+    {
+        EXPECT_EQ(cpuMatches[i].queryIdx, oclMatches[i].queryIdx) << "at index " << i;
+        EXPECT_EQ(cpuMatches[i].trainIdx, oclMatches[i].trainIdx) << "at index " << i;
+        EXPECT_NEAR(cpuMatches[i].distance, oclMatches[i].distance, 1e-3f) << "at index " << i;
+    }
+}
+
 }} // namespace