Fix reg shift python sample

[shiffiH]

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• I agree to contribute to the project under Apache 2 License.
• 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

[shiffiH]

This PR fixes the Python sample modules/reg/samples/reg_shift.py by:

• Preferring the canonical cv.reg.* API, with a fallback to underscore-style bindings for older versions.
• Avoiding nested mapper construction, which was reported to cause segfaults.

Fixes #1935

[shiffiH]

Hi, this PR is ready for review. CI is currently awaiting approval to run workflows for the fork.