Services no longer receive the concrete account store. on_command now takes
&mut dyn Store + &dyn NetView (both defined in fedserv-api); the engine's Db
and Network implement them. Reads hand back plain views (AccountView,
ChannelView, ...) carrying only non-secret fields, so the event log, gossip,
and credential material (password hash, SCRAM verifiers) are unreachable from
a module. All command modules ported; behaviour unchanged.
Move NetEvent/NetAction/RegReply/Protocol and Sender/ServiceCtx into a
dependency-free fedserv-api crate that a module builds against. Core keeps
referring to them through re-export shims at crate::proto and
crate::engine::service, so no consumer changes. Groundwork for external
service and protocol modules.
