Add real-D1 coverage for core migrations and queries

[ascorbic]

Goal

Core migration and query behavior should be tested against real D1 before 1.0.

Scope

Extend the existing D1 test patterns to packages/core so migration and query bugs caused by D1-specific behavior are caught before release.

Acceptance criteria

• packages/core has a real-D1 test path for migrations.
• Representative query helpers run against real D1.
• The setup works in CI or has a documented opt-in path with clear coverage expectations.
• Tests cover at least the classes of D1-specific bugs seen in previous production issues.
• The test utilities make it hard to accidentally run only sqlite/postgres for D1-sensitive code.