Add a --shared-dir / SOULSEEK_SHARED_DIR option and a SOULSEEK_DOWNLOAD_DIR
env for the download directory. The shared directory is resolved and
validated up front (tilde-expanded, must exist and be a directory); a
misconfigured value is a warning, not a fatal error.
The library gains an optional shared_directory on ClientSettings/Client
with a getter, keeping soulseek-rs-lib dependency-free. Resolution lives
in a new soulseek-rs/src/directories.rs with unit tests.
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.
michel