Delivery vocabulary shared by Connection (stamping + direct turn delivery) and Client (turn consumption).

The ordering key is a receiver-assigned contiguous per-turn ordinal, stamped at the single sequential demux point before any per-session fan-out. It is never read from the peer's _meta: arrival order is not stable across a reattach onto a new connection, and a peer that supplied the key could manufacture a gap to force the degraded path. Every drop/gap decision emits telemetry rather than dropping silently, and the per-turn namespace that scopes the ordinals is receiver-minted too, so a peer reusing a turn id cannot alias a stale turn's straggler forward.

Message shapes (the direct turn-delivery channel)

For a turn opened by a session/prompt submitted via Connection.async_request/6 (reply_to = {:owner, owner, tag}), the Connection sends owner three message shapes directly, ALL from the Connection process itself — the same single-sender operation deliver_outcome/2 already uses for the terminal result — so BEAM's per-sender-per-receiver FIFO guarantees the owner's mailbox order equals demux order equals wire order:

• {:acp_turn_update, tag, ordinal, notification} — one per session/update notification belonging to the turn's session, stamped with a contiguous per-turn ordinal starting at 0.
• {:acp_turn_end, tag, count} — sent exactly once, immediately before the terminal {:acp_result, tag, outcome}; count is the number of :acp_turn_update messages the owner should have received (ordinals 0..count-1).
• {:acp_result, tag, outcome} — the terminal delivery (Connection.async_request/6's outcome).

Because the Connection is the single sender of all three shapes, a consumer needs no settle timer and no reorder buffer on this path: delivered < count observed at the terminal result is a definite, synchronous signal of dropped updates, never a race.

The stamp (broadcast path)

t/0 is the per-notification ordering key threaded onto Connection.Ctx.delivery_stamp for every session/update dispatch (nil for anything else) — for the multi-sender broadcast path (subscribe/3, custom session_update/2 overrides) that still fans out through per-notification dispatch tasks and therefore needs an explicit key to reconstruct order. turn is the receiver-minted turn token (nil when the update is out-of-turn — no open prompt turn for the session on this connection); ordinal is the contiguous per-(session, namespace) stamp assigned at the demux point; rx_seq is the connection-wide monotone frame stamp (used by the separate cancel/prompt wire-order latch).