Rebrand the daemon and workspace: the binary is now `echo`, every crate
is `echo-*` (`echo-api` and the module crates), Rust imports use
`echo_*`, the gRPC proto is `proto/echo.proto` with package `echo.v1`,
and the log filter, gossip peer-name default, and on-disk store default
(`echo.db.jsonl`) follow. Docs updated throughout. No behaviour change;
crate directories and the config schema are untouched.
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.
