Negative RGB mapping indices are accepted

`prepare_for_display()` validates only the upper bound of `rgb_mapping` indices. Negative indices therefore pass validation and use NumPy negative indexing, silently selecting channels from the end of the image.

For a 4-channel image, `rgb_mapping=[-1, 0, 1]` is accepted and maps channel 3 into red instead of rejecting the invalid mapping.

Locate the prepare_for_display() entry point and inspect how rgb_mapping indices are validated before NumPy indexing. Verify that negative indices are rejected while valid mappings continue to work, and add or update focused tests for the four-channel example.

Written by the indexing model from the issue text.