Extract signal-client: socket path and frame iteration #9

default_socket_path is duplicated in signald/src/main.rs and terminal-garden/src/main.rs, with a README table and a pinned test in each binary keeping the copies honest. A third renderer makes a third copy.

Done when:

• crates/signal-client/ holds socket-path resolution and a Frames iterator over the socket yielding wire::Frame.
• Both renderers use both parts; signald uses it for default_socket_path.
• One socket-path test replaces the two duplicates.