docs(loyalty): fix dangling reference, minor-unit amount, breakdown invariant, and code registries

[vishkaty]

Problem

Four small loyalty corrections:

1. Dangling example reference. The main example says it builds on a "store loyalty card example" from the checkout Eligibility Verification at Completion section, which contains only a student-verification example.
2. Minor-unit amount reads as major units. The store-card discount example applies amount: 10 ($0.10 in minor units) while the surrounding text describes a $10 benefit; sibling examples use minor units (1000).
3. Breakdown has no sum invariant. earning_forecast.breakdown amounts have no statement tying them to earning_forecast.amount, unlike discount allocations and totals.
4. New codes absent from the examples registries. membership_benefit_eligible and membership_benefit_ineligible are defined in a normative table but not in the info_code/warning_code examples arrays that tooling treats as the standard-code registry.

Fix

• loyalty.md: reword to build on the eligibility-verification pattern and introduce the store-card scenario inline; correct the four discount/allocation amounts to 1000.
• loyalty.json: state that the breakdown amounts MUST sum to earning_forecast.amount.
• info_code.json / warning_code.json: append the two codes.

Verification

The four amount changes sit inside an extract=$.loyalty example, so no totals are affected; the points-earning breakdown (10 + 20 = 30) is untouched. ucp-schema lint source/ and validate_examples.py pass (282, unchanged).