Download immutable artifact bytes

Full retained binary download, at most 67108864 bytes with a 60000 millisecond overall budget. No guest or execution calls. Shared read slots/pins remain held through stream and pending local I/O completion. Backpressure queues at most one 64-KiB payload chunk. Original authority and current row/generation/expiry are checked before enqueue; fresh exact computer scope is required after one second or one MiB, whichever first. Revocation cannot retract already delivered or queued bytes. Errors after headers terminate the stream; a chunk that fails its check or cannot be read also withdraws the artifact, if storage can record that: retry it, and capture a new one only if the retry answers 404. Content-Length is exact, Content-Disposition is a generated art_<hex>.bin attachment, with nosniff, no-store and Accept-Ranges:none. Incoming Range is ignored with a full 200 response. No query parameters.

Requires the member role or higher.