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.
The one command handler (ROLL/EXROLL/CALC/EXCALC all route to it) and
its formatting helpers move to roll.rs; lib.rs keeps the dispatcher, the
expression-evaluator module, and its tests. No behaviour change.
