Merge pull request #26865 from amane-ame:dxt_hal_rvv

Add RISC-V HAL implementation for cv::dft and cv::dct #26865

This patch implements `static cv::DFT` function in RVV_HAL using native intrinsic, optimizing the performance for `cv::dft` and `cv::dct` with data types `32FC1/64FC1/32FC2/64FC2`.

The reason I chose to create a new `cv_hal_dftOcv` interface is that if I were to use the existing interfaces (`cv_hal_dftInit1D` and `cv_hal_dft1D`), it would require handling and parsing the dft flags within HAL, as well as performing preprocessing operations such as handling unit roots. Since these operations are not performance hotspots and do not require optimization, reusing the existing interfaces would result in copying approximately 300 lines of code from `core/src/dxt.cpp` into HAL, which I believe is unnecessary.

Moreover, if I insert the new interface into `static cv::DFT`, both `static cv::RealDFT` and `static cv::DCT` can be optimized as well. The processing performed before and after calling `static cv::DFT` in these functions is also not a performance hotspot.

Tested on MUSE-PI (Spacemit X60) for both gcc 14.2 and clang 20.0.

```
$ opencv_test_core --gtest_filter="*DFT*"
$ opencv_perf_core --gtest_filter="*dft*:*dct*" --perf_min_samples=30 --perf_force_samples=30
```

The head of the perf table is shown below since the table is too long.

View the full perf table here: [hal_rvv_dxt.pdf](https://github.com/user-attachments/files/18622645/hal_rvv_dxt.pdf)

<img width="1017" alt="Untitled" src="https://github.com/user-attachments/assets/609856e7-9c7d-4a95-9923-45c1b77eb3a2" />

### 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
- [ ] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] 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/core/src/dxt.cpp

@@ -844,6 +844,17 @@ DFT(const OcvDftOptions & c, const Complex<T>* src, Complex<T>* dst)
     Complex<T> t;
     T scale = (T)c.scale;
 
+    if(typeid(T) == typeid(float))
+    {
+        CALL_HAL(dft, cv_hal_dft, reinterpret_cast<const uchar*>(src), reinterpret_cast<uchar*>(dst), CV_32F,
+                 c.nf, c.factors, c.scale, c.itab, c.wave, c.tab_size, c.n, c.isInverse, c.noPermute);
+    }
+    if(typeid(T) == typeid(double))
+    {
+        CALL_HAL(dft, cv_hal_dft, reinterpret_cast<const uchar*>(src), reinterpret_cast<uchar*>(dst), CV_64F,
+                 c.nf, c.factors, c.scale, c.itab, c.wave, c.tab_size, c.n, c.isInverse, c.noPermute);
+    }
+
     if( c.useIpp )
     {
 #ifdef USE_IPP_DFT