test: persistence e2e and registration-relogin e2e; lint clean

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.

soulseek-rs/src/lib.rs

@@ -0,0 +1,6 @@
+//! Library surface of the TUI binary, exposed for integration tests.
+//! The binary itself compiles these modules directly (see `main.rs`).
+
+pub mod cli;
+pub mod models;
+pub mod persist;