feat(studies): add approval_at_optimal rated-method helper (split 2/3 of #56)

[endolith]

Split 2 of 3 from #56, stacked on #52 (base: cursor/issue-10-simulation-api-44e5), sibling of #81.

Problem: rated_methods mappings repeat the same lambda (approval(approval_optimal(utilities), tiebreaker)) in every Merrill-style script.

Approach: promote the private _approval_at_optimal wrapper to public approval_at_optimal(utilities, tiebreaker="random"), add a docstring, export it from elsim.studies, and switch merrill_1984_comparison_methods to use it. Existing tests cover it through merrill_1984_comparison_methods.

Commits:

• b4f56e2 feat(studies): promote approval_at_optimal to a public rated-method helper

Also drops a stray backslash in a condorcet_metrics docstring while in the file.

— Skales

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