Edit Collection Members

Incrementally assemble a draft collection: add/remove member memories and/or update storefront metadata. Each call republishes a new STAGED manifest version (immutable audit); the collection stays non-consumable until /promote. Members may reference staged OR published memories while drafting — /promote is what enforces that they are all published. Body: {add?:[{id, version, kind?}], remove?:[id, ...], title?, description?, tags?, visibility?, price?, license?}