Make compound policies definition-driven in Policy Workbench #8684

Part of #8160.

Goal

Prepare Policy Workbench to support compound policies with more than one child.

The backend already supports parentPolicyKey, compositeChildren[] and transactional compound saves.

This issue only coordinates the frontend work.

Child issues

• Add compound policy write methods to the policies store #8686 — Add compound policy write methods to the policies store
• Make the Policy Workbench compound helper use policy definitions #8687 — Make the Policy Workbench compound helper use policy definitions

Order

#8686
  ↓
#8687

Done when

This issue is complete when both child issues are complete and existing compound policies still work without policy-specific branches being added to the shared Workbench engine.

Out of scope

Signature Rejection UI is tracked by #8685.