sorts: type shrink shell sort for comparable items

[taljeon]

Summary

• bind shrink_shell_sort.shell_sort to a Comparable protocol while preserving its list[T] -> list[T] in-place contract
• add string, mixed numeric, and non-comparable doctest coverage
• include shrink shell sort in the shared comparable-sort success and TypeError regression matrix

Part of #15234. This intentionally does not close the tracking issue.

Validation

• python3 -m doctest -v sorts/shrink_shell_sort.py — 6 passed
• python3 -m pytest tests/test_sorts.py sorts/shrink_shell_sort.py --doctest-modules -q — 245 passed
• ruff 0.16.7 check . — passed
• ruff 0.16.7 format --check . — 1,546 files already formatted
• git diff --check — passed

AI assistance

OpenAI Codex (gpt-5.6-sol) assisted with the focused type/test update and verification. I reviewed the complete two-file diff and the validation results above.

Describe your change

• Add an algorithm?
• 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.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

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

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