Start a run

Scope says where an attribute's values live on a product that has variants; on a product without variants the attribute is the product's own either way. Moving to `variant` copies each product's value onto its variants that hold none (a variant's own value is never overwritten), then removes the product's value; a product without variants keeps its value as its own. Content Fabric writes per product cannot move to `variant`. Moving to `product` takes the value a product's variants agree on onto the product and removes it from the variants. Agreement is strict: every variant holds the same value, or none does. If any product disagrees, nothing moves and the answer is 409 with one `errors` entry per product (at most 20). Every value written is recorded in its history against the API key, and the change appears in the brand's activity. A move rewrites up to 25,000 variants in one transaction, so it sits on the `job` budget.