Fix modprodnet distributed pair activation

[bernalde]

Summary

• Fixes the distributed modprodnet site-pair inactive disjunct so it represents "not both sites active" instead of forcing both sites inactive.
• Applies the same correction to the quarterly distributed formulation.
• Adds focused regression coverage for all public modprodnet cases, Big-M/Hull reformulation smoke tests, invalid case handling, and the corrected pair-inactive semantics.

Tests run

• /home/bernalde/.pixi/bin/pixi run pytest tests/test_modprodnet.py -v --tb=short
  • Result: 24 passed in 4.74s
• /home/bernalde/.pixi/bin/pixi run pytest tests/test_module_imports.py -v --tb=short
  • Result: 68 passed in 6.43s
• /home/bernalde/.pixi/bin/pixi run python - <<'PY' ... PY
  • Direct GDPopt GLOA check for default modprodnet using GAMS-local role solvers (nlp=ipopth, mip=gurobi, minlp=dicopt, local_minlp=dicopt), 60s time limit.
  • Result: status ok, termination optimal, objective/lower/upper bound 3592.9243356537468.
• /home/bernalde/.pixi/bin/pixi run test
  • Result: 308 passed, 1 skipped in 26.44s
• /home/bernalde/.pixi/bin/pixi run lint
  • Result: exit 0. Black, critical flake8, and typos passed; the repository's existing non-fatal flake8 --exit-zero style inventory was printed.
• git diff --check
  • Result: no whitespace errors.

Notes

• I did not run the full solver-backed benchmark campaign. A narrow gdplib-benchmark run --instances modprodnet --strategies gdpopt.gloa --timelimit 60 --run-id issue72_gloa_probe --solver-profile gams-local probe produced an optimal per-case result, but the benchmark CLI exited during summary generation because generate_benchmark_summary_all is not available in this checkout. The direct GDPopt GLOA solve above was used as the clean solver-backed verification.
• Issue Plan PyPI release workflow and Pixi platform support #104 is closed release/Pixi follow-up work; this PR intentionally does not change Pixi platform or release metadata.

Closes #72