ptcloud , photo test suit cleanup - #29742
## Test suite cleanup
### Given real assertions
- **ptcloud** — `HugeSceneGrowthTest`: zero assertions, including a `// Reset check` comment followed by no check.
- **ptcloud** — `PointCloud.SaveBadExtension`: passed an empty vertex set, so it exited at the empty-input guard and never reached the extension code it is named for.
- **ptcloud** — new `PointCloud.SaveEmptyVertices`: covers the early-return branch the above was hitting by accident.
### Moved
- **photo** — `Photo_Denoising.speed` → `perf/perf_denoising.cpp`: a `getTickCount` + `printf` stopwatch in the accuracy suite, asserting nothing, costing 393 ms per run.
### Library fixes found while doing the above
- **ptcloud** — `findPlanes` now converts 3-channel input instead of reshaping it: `Mat_<Vec4f>::operator=` reshapes when depths match, so a 320×240 `CV_32FC3` input silently became 240×240.
- **ptcloud** — new `RGBD_Plane.regression_3channel_matches_4channel`: nothing covered the documented 3-channel path, since all 40 `RgbdPlaneGenerate` cases feed `CV_32FC4`.
### 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.
- [x] The feature is well documented and sample code can be built with the project CMake
Dedicated pointcloud module #29224
OpenCV contrib: https://github.com/opencv/opencv_contrib/pull/4134
### 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
