Add tax incidence properties

[alexanderthclark]

Summary

• Add explicit tax incidence properties to Equilibrium/Market for introductory per-unit tax and subsidy analysis.
• Expose the tax wedge, consumer and producer burden amounts, and burden shares using the existing p_consumer / p_producer tax machinery.
• Add a short docs example to the existing Basic Tax Example showing how to inspect the burden split.

Behavior

• tax_wedge reports p_consumer - p_producer.
• consumer_tax_burden and producer_tax_burden compare tax/subsidy prices against the no-intervention market price.
• consumer_tax_share and producer_tax_share divide those signed burdens by the signed tax.
• No-tax cases return zero incidence values, keeping price controls and world-price behavior out of scope.

Validation

• pytest -q tests/test_equilibrium.py tests/test_world_price.py passed: 12 passed, 1 warning.
• pytest -q passed: 141 passed, 2 warnings.

Notes

This PR is intentionally limited to closed-economy per-unit tax incidence. It does not change tariff behavior, plotting, notebooks, or version numbers.