Add price-control gap properties

[alexanderthclark]

Summary

• Add first-class market gap properties to Equilibrium/Market for Econ 101 price-control analysis.
• Expose quantity demanded, quantity supplied, shortage, and excess-supply surplus at the relevant market price.
• Add compatibility aliases for excess_demand_quantity and excess_supply.
• Update the price-controls tutorial to use the new properties instead of manually computing gaps from the curves.

Behavior

• Binding ceilings still trade at quantity supplied and now expose the resulting shortage.
• Binding floors still trade at quantity demanded and now expose the resulting surplus.
• Non-binding controls and free-market equilibria return zero shortage and zero surplus.
• Welfare properties such as consumer_surplus, producer_surplus, and total_surplus are unchanged.

Validation

• pytest -q tests/test_equilibrium.py passed: 7 passed, 1 warning.
• pytest -q passed: 140 passed, 2 warnings.

Notes

This PR intentionally avoids plotting changes, notebook regeneration, docs build artifacts, and version changes.