Adds a lib target (soulseek_rs_tui) exposing cli/models/persist so
integration tests can drive the persistence pipeline: state and config
survive a simulated restart through the real files, with the incomplete
download flagged for auto-resume. The lib e2e suite gains a
register-then-relogin test against soulfind proving the stored-credential
flow's server assumptions. cargo clippy --all-targets and cargo fmt are
clean.
The main TUI now loads state on startup — search history restored as
inert entries, saved chat rooms rejoined on the server, and incomplete
downloads (including failed/timed-out ones) automatically re-enqueued —
and saves a diffed snapshot each loop tick and on exit, so only changed
files are rewritten.
michel