Refactor Monopoly solver helpers

[alexanderthclark]

Summary

• Refactor Monopoly._solve() into smaller private helpers for candidate generation, domain checks, profit evaluation, and output assignment.
• Preserve the existing solver behavior and public API, including q, p, profit, __repr__(), and _repr_latex_().
• Add regression tests for zero-output fallback and fixed-cost behavior so this cleanup does not accidentally introduce shutdown logic or other economic behavior changes.

Validation

• pytest -q tests/test_monopoly.py passed: 9 passed.
• pytest -q passed: 139 passed, 2 warnings.
• PYLINTHOME=/private/tmp/pylint-home python -m pylint freeride/monopoly.py passed: 10.00/10.

Notes

This is intended as a behavior-preserving readability cleanup only. It does not change docs, version numbers, or monopoly economics.