Cycle detection in CI (madge or knip classMembers)

[jorgeraad]

Backs #727 (break circular deps in core barrels) with an enforcement check so cycles can't reappear after they're fixed.

Plan: pick a tool (madge has a clean --circular mode; knip's classMembers/types coverage may already report cycles indirectly), wire into the dead-code workflow, baseline current cycles, fail on new ones.