cmake: fix#29028 macOS dynamic framework RPATH for relocatable install #29396
### Problem
When building OpenCV as a dynamic framework on macOS (`-DAPPLE_FRAMEWORK=ON -DBUILD_SHARED_LIBS=ON`), installed binaries contain an absolute `LC_RPATH` pointing to the build tree, making the install non-relocatable. Moving the install directory or deploying to another Mac causes binaries to crash on launch.
### Root cause
`cmake/OpenCVInstallLayout.cmake` unconditionally sets `CMAKE_INSTALL_RPATH` to `${CMAKE_INSTALL_PREFIX}/${OPENCV_LIB_INSTALL_PATH}` for all platforms. Additionally, `cmake/platforms/OpenCV-Darwin.cmake` was empty and missing the `@executable_path`/`@loader_path` linker flags that the iOS toolchain already sets for the same `APPLE_FRAMEWORK AND BUILD_SHARED_LIBS` case.
### Fix
- Guard `CMAKE_INSTALL_RPATH` in `OpenCVInstallLayout.cmake` to use relocatable `@executable_path` tokens for Apple framework builds
- Add equivalent linker flags to `OpenCV-Darwin.cmake`, mirroring `platforms/ios/cmake/Modules/Platform/iOS.cmake`
Fixes#29028
### 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
- [ ] 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
Add support for QNX #25832
Build and test instruction for QNX:
https://github.com/chachoi-world/qnx-ports/blob/main/opencv/README.md
### 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
- [ ] 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
Anshu
James Choi
Alexander Alekhin
Maksim Shabunin