Add integration test suite for all modules

[jc-macdonald]

Summary

Current test coverage threshold is 40% and the only test is a smoke test (import trade_study). All 8 source modules have real implementations but zero tests.

Modules Needing Tests

Module	Key functions to test
scoring.py	score() dispatcher for crps, wis, interval, energy, brier, coverage, rmse, mae; coverage_curve()
pareto.py	extract_front(), pareto_rank(), hypervolume(), igd_plus()
stacking.py	stack_bayesian(), stack_scores(), ensemble_predict()
design.py	build_grid() full factorial + LHS, screen(), reduce_factors()
runner.py	run_grid(), run_adaptive()
study.py	Study multi-phase orchestration, top_k_pareto_filter()
io.py	save_results() / load_results() round-trip
protocols.py	Protocol conformance checks

Acceptance Criteria

• Each module has at least one @pytest.mark integration test
• Coverage threshold raised to ≥80%
• Tests run in CI via just test