Explain Policy

Compile a policy and return the plan, without dispatching anything.

Master-key gated, and deliberately so: the response enumerates the policy's targets, which is exactly the information a policy exists to keep off the wire. It is a management surface, not a caller-facing one.

Accepts an unsaved spec as well as a saved name, so a form can validate what the operator is about to save. The response includes dropped candidates with reasons, which is the part that catches a "failover" policy that has quietly compiled down to a single attempt.