[API] Add capability-based public contract tests

[lrnv]

Scope

Build contract tests for the public API and map existing generic tests to those contracts. The goal is confidence in stable behavior without testing the full family-by-operation Cartesian product.

Approach

• define small representative fixtures for universal, absolutely continuous, singular, conditionable, Rosenblatt-invertible, and fittable cases;
• test public entry points rather than incidental internal helpers;
• verify return types/shapes, boundaries, errors, round trips, and deterministic invariants;
• distinguish generic contract tests from family-specific numerical regressions;
• reuse the dispatch-path inventory from [Housekeeping] Rationalize tests around dispatch paths #424 to avoid redundant compilation-heavy coverage;
• ensure exported operations are not present only nominally but are reached through tests.

Acceptance criteria

• a traceable public-operation-to-test matrix;
• contract coverage for each supported capability category;
• explicit tests for unsupported operations and singular cases;
• Rosenblatt/inverse and conditioning consistency tested where promised;
• fitting tested only for families and methods that declare support;
• redundant expensive combinations removed only after equivalent contract/path coverage is demonstrated;
• before/after test timing reported when the suite structure changes materially.