test(review): add the four new-defect fanout reviewer-diversity eval cases #12143

Parent #7584's reviewer-diversity eval (Q134, Q160, D101; approved as recommended by the owner on 2026-10-15, poteto-review decision row 99) runs 12 fixtures under four arms. #12139 adds the arm wiring, the graders and four reused-scaffold fixtures, and describes the case layout under plugins/review/evals/. This slice adds the four new defect fixtures from the approved Q160 design, in that layout.

Done when

• plugins/review/evals/ gains four fanout-diversity-* fixtures, each with a scaffold that plants one defect: a race on shared state, a resource leak on an error path, a caller left on an old contract, and a test that cannot fail.
• Each fixture has a ground-truth file that names its planted defect and that defect's axis (spec when the change fails its stated purpose, else standards), in the shape test(review): add the fanout reviewer-diversity eval wiring and its four reused-scaffold cases #12139 uses.
• Each fixture has one case directory per arm and the three graders, each grader file following its template in plugins/review/evals/fanout-diversity.md and differing from test(review): add the fanout reviewer-diversity eval wiring and its four reused-scaffold cases #12139's only in the planted-defect text, and each grader has a samples/<grader>.json with at least one pass and one fail entry.
• Each case uses schema_version: "1.1" in case.yaml and sets runs: 3, as test(review): add the fanout reviewer-diversity eval wiring and its four reused-scaffold cases #12139's cases do.
• The diff edits no file under plugins/review/skills/ and no file under plugins/review/agents/.
• A .changes/review/*.md fragment with bump: none describes the cases.
• The PR's CI is green.

Out of scope

• The decoy fixtures (a sibling slice), and running the eval.

Parent: #7584