Append an item. Auto-forwards if wired.
The local buffer append is a no-op when no subscriber is registered, but auto-forwarding always fires so wired events reach the main event log regardless of subscription state.
Items are stored as (stamp, item) tuples where stamp is a
monotonic counter from the owning mux. Stamps are stripped by
the default cursors; raw stamped tuples are visible on _items.
