test: replace broad matrices with focused contracts

[upadhyan]

Summary

• replace broad Cartesian and smoke-test matrices with focused behavioral contracts
• add direct coverage for feature/weight boundaries, multi-output behavior, TAO outcomes, forest bootstrap alignment, exported routing, and dataset invariants
• retain narrow sklearn reference skips while keeping native assertions active

Verification

• Python: 277 passed, 2 skipped
• C++: 30/30 passed
• Ruff and focused mypy checks pass
• production branch coverage: 92%
• local focused mutation audit: 338 killed, 173 survived among 511 tested mutants

Mutation testing remains local-only; this PR makes no workflow, dependency, or production-code changes.