Generalize cuVS Bench plugin CLI

[nvzm123]

Note

Draft 1 of 5 in the Lucene backend split. This PR contains only generic
cuVS Bench plugin/CLI infrastructure and has no Lucene-specific runner path.

Summary

• let registered backends declare a prompt default and interpret failed results;
• add generic --backend selection while retaining --backend-config compatibility;
• preserve prior measurements for failed, skipped, and dry-run records,
  including partial sweeps whose parameter schema changed between runs;
• persist a stable raw-row search-plan identity while keeping frontier schemas unchanged;
• keep the C++ backend's existing cuvs_cagra default explicit; and
• raise the Click minimum required for reliable command-line parameter-source detection.

Validation

• focused CLI, export, registry, and C++ backend tests: 50 passed, 2 deselected;
• Python compilation and git diff --check: passed.

The two deselected pre-existing C++ dry-run cases write beneath read-only
/data. Ruff was unavailable in the local environment.