v0.7.4 — Fresh install agnostic contract demonstrated on real MySQL

Adds tests/Integration/Mysql/ suite covering the agnostic install contract
for int/uuid/ulid against MySQL 8 via a new opt-in MySQL CI job.

Retires the unproven repair_article_customer_id_columns migration and its
stub. Pre-v1.0 dev-main does not promise schema upgrades; consumers use
migrate:fresh.

See docs/2026-05-09-fresh-install-agnostic-mysql.md for the contract.
PR #21.