types: finish the mypy ratchet across experimental, models, and preprocessing

[shivamlalakiya]

Summary

• Annotates the last 129 unannotated functions in philanthropy.experimental, philanthropy.models, and philanthropy.preprocessing, the three subpackages issue Finish the mypy ratchet: annotate model_selection, experimental, cli, models, preprocessing #166 tracked as remaining.
• fit returns a per-class TypeVar bound to the class instead of a string-literal forward ref, so a subclass's fit no longer reports its parent's type. __sklearn_tags__ returns sklearn's public Tags dataclass (scikit-learn>=1.6, the declared floor).
• With every subpackage covered, collapses [[tool.mypy.overrides]] into a top-level disallow_untyped_defs = true in pyproject.toml. Closes Finish the mypy ratchet: annotate model_selection, experimental, cli, models, preprocessing #166.
• CHANGELOG ### Typing entry updated in place (matching the pattern from types: annotate metrics, utils, inspection and visualisation, and start the mypy ratchet #167/types: annotate model_selection and extend the mypy ratchet #172/types: annotate cli.py and extend the mypy ratchet #173) rather than appended, since it's the same ongoing item reaching completion.

No behavior change: every edit is a type annotation. Verified with make ci, make riskcov, and the ratchet probe from #166 (appending an unannotated function now fails mypy, confirmed then reverted).

This PR only covers the mypy-ratchet item. A pass over plan.md (gitignored, not part of this diff) found that the rest of the JOSS-prep checklist's code-shaped items — make_donor_panel, the three CI-executed notebooks, the real-data docs page, the "which estimator" table, YAML issue forms, the coverage-badge link, pypi-smoke.yml — were already shipped in earlier PRs (#161, #162, #165, #167, #170, #172, #173). This is genuinely the last code-only item left in that list; everything else remaining is non-code (preprint, Discussion post, outreach).

Test plan

• python -m mypy philanthropy — success, no issues, 50 source files
• Ratchet probe from Finish the mypy ratchet: annotate model_selection, experimental, cli, models, preprocessing #166: an unannotated function appended anywhere in philanthropy fails mypy with disallow_untyped_defs; reverted after confirming
• make ci (flake8, mypy, doctests, full test suite) — green
• make riskcov — 97% branch coverage over the risk-tier subtree, above the 93% floor