An outer surface that exposes the store's public facade to a protocol.

A transport is replaceable because it is the edge: it owns request parsing, authentication, rendering, and its own lifecycle, and it reaches the store only through the public AgentDb facade. It never touches storage or inference directly, so a store behaviour change does not ripple into wire formats and a transport change cannot alter what the store guarantees.

A transport is optional. enabled?/0 decides whether its children are composed at all, and child_specs/1 supplies the rest.