Move or rename. Refuses to replace an existing path unless overwrite.

Replaying the same key with the same request within 24 hours returns the first result and never repeats the effect. A refusal is not kept: a request refused before it ran (no_capacity, busy, a limit) runs afresh when sent again with its key. The SDKs send one automatically; the server makes one when absent and returns it in the idempotency-key response header.