Pull the JSON response-file layer out of cli/mod.rs into a new
cli/exchange.rs:
- cli_*_path helpers (snapshot, library, server, search, audio_device)
— XDG_RUNTIME_DIR-aware path resolvers
- write_cli_snapshot
- write_library_cli_response, write_server_list_cli_response,
write_search_cli_response, write_audio_device_cli_response
- read_*_cli_response_blocking pollers (private to cli)
- print_*_cli_stdout wrappers (private to cli; thin "JSON or human"
switches around the presenter functions)
Externally-visible names (cli::write_cli_snapshot, the four
write_*_cli_response, the path helpers) preserved via `pub use
exchange::*` re-export at the cli root. Internal-only functions
demoted to `pub(super)`.
cli/mod.rs: 895 → 696 LOC. Behaviour-preserving — same file paths,
same atomic write-via-tempfile pattern, same poll intervals + ready
predicates.
Psychotoxical