Fix stranded gas learning reformulation

[bernalde]

Summary

• Replaced the stranded_gas module-learning logarithm inside the module-existence disjunct with a finite purchase-count selector over the existing 0..5 purchase limit.
• Made absent module types force zero purchases, and marked exactly-one stranded_gas disjunctions with xor=True.
• Added focused stranded_gas semantic and gdp.bigm/gdp.hull transformation tests, then regenerated model-size reports.

Tests run

• /home/bernalde/.pixi/bin/pixi run pytest tests/test_stranded_gas.py tests/test_module_imports.py -v --tb=short - passed, 86 passed in 48.52s.
• /home/bernalde/.pixi/bin/pixi run test - passed, 340 passed and 1 skipped in 93.33s.
• /home/bernalde/.pixi/bin/pixi run lint - passed. Black and critical flake8 passed; the repository's --exit-zero flake8 inventory still reports existing style issues outside this PR.
• /home/bernalde/.pixi/bin/pixi run pytest tests/test_stranded_gas.py -v --tb=short - passed after formatting, 12 passed in 33.98s.
• git diff --check - passed.
• /home/bernalde/.pixi/bin/pixi run gdplib-benchmark preflight --instances stranded_gas --strategies gdp.bigm gdp.hull --solver-profile gams-baron --timelimit 60 --build-models --check-solvers - passed.
• /home/bernalde/.pixi/bin/pixi run gdplib-benchmark run --instances stranded_gas --strategies gdp.bigm --solver-profile gams-baron --timelimit 60 --run-id issue76_bigm_baron_60s --fail-fast --no-summary - passed with 1 result row and 0 failures; BARON reached maxTimeLimit with a finite lower bound instead of the previous infeasible result.

Notes

• I did not run the full multi-strategy, 3600-second solver-backed benchmark campaign.
• Follow-up issue Plan PyPI release workflow and Pixi platform support #104 is closed and release/Pixi-policy focused, so no packaging changes are included here.

Closes #76