Ensure default rounds cover active memberships

[KyleHerndon]

Summary

• Treat unspecified min_episodes_per_entrant as one appearance per entrant when calculating default pool episode counts.
• Add a round-start regression covering five active memberships with a two-agent variant, ensuring all memberships appear in the scheduled games.

Validation

• Lint: no lint command is configured in pyproject.toml, .github/workflows, or README.
• uv run pytest tests/test_commissioner_strategies.py -q passed: 59 passed.
• uv run pytest -q was attempted; it printed the strategy-test progress and then hung in the remaining tests, so it was interrupted.