Append-only conversation item log with live-tail-no-replay streaming.

Wraps a Raxol.Agent.Conversation.Store (durability) with an in-process subscriber fan-out (liveness). The stream itself keeps NO replay buffer -- all history lives in the store. This is the omnigent model: durability in the store, the stream is ephemeral pub/sub.

Exact snapshot/live-tail partition

subscribe/3 returns a snapshot AND registers the caller as a subscriber in a single serialized GenServer.call. Because append/3 is also a serialized call, the two never interleave: a subscribe processed before an append excludes those items from its snapshot but receives them on the live tail; a subscribe processed after an append includes them in the snapshot and is NOT re-sent them. So a reconnecting client sees every item exactly once with no gap and no dupe -- belt-and-suspenders deduping by Item.id is then trivial.

Live items arrive as {:conversation_item, conversation_id, item} messages. Subscribers are monitored; their subscriptions are dropped automatically when they exit.

Usage

{:ok, log} = Log.start_link(name: MyLog)
{:ok, %{snapshot: history, last_seq: cursor}} = Log.subscribe(log, "conv-1")
{:ok, [item]} = Log.append(log, "conv-1", %{type: :message, data: %{content: "hi"}})
# the subscriber's mailbox now holds {:conversation_item, "conv-1", item}

Reconnect by passing the last seq seen: Log.subscribe(log, conv, after: cursor).