sorts: make merge_insertion_sort generic over Comparable items

[modusensus]

Describe your change

• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
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• All functions have doctests that pass the automated testing.
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Part of #15234 — this generalizes merge_insertion_sort (and its helpers binary_search_insertion, merge, sortlist_2d) so they accept any mutually comparable items, mirroring the Comparable Protocol pattern already merged for insertion_sort, tree_sort, shell_sort and others. All existing doctests are unchanged, including the permutations test.

[cclauss]

ON HOLD: Our focus is on merging or closing old pull requests before October 1st.

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