Upload one chunk of a build context: at most 1 MiB, named by its SHA-256.

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.