Adds soulseek-rs-lib/tests/e2e.rs, which drives the client's public API
(connect, login, search) against a real Soulseek server. The harness is
server-optional so cargo test stays green everywhere: it uses
SOULSEEK_TEST_SERVER if set, otherwise spawns a soulfind binary (SOULFIND_BIN
or a soulfind/bin/soulfind checkout in a parent dir) on an ephemeral port with
a throwaway database, and skips with a notice when no server is available.
Uses only std and the library's public API — no new dependencies. Documents how
to run it (including the macOS soulfind SQLite workaround) in the README.
