Set the absolute network quantity of one variant.

• Response: HTTP 200 with the same variant_id, location_id and available_quantity, with the same types. Additional response fields are ignored.
• Order of work: apply the write first, then acknowledge it.
• No version: the payload carries no timestamp or version, so a late write is not detected.
• Retries: SKUU retries a network failure, 429 and any 5xx. A write that still failed is attempted again on later sync cycles, also after a 404 or an incorrect receipt.