Extend the existing CI workflow with an ubuntu-only "End-to-end (soulfind)"
job that builds soulfind from source (LDC + `dub build :server`), points
SOULFIND_BIN at it, and runs the e2e suite with SOULSEEK_E2E_REQUIRED=1.
Also widen the clippy job to the whole workspace.
The e2e harness gains SOULSEEK_E2E_REQUIRED: when set, a missing/unstartable
server is a hard failure instead of a silent skip, so CI genuinely exercises
the client end-to-end. Add e2e coverage for two existing behaviours a single
server can verify: wrong-password rejection and concurrent logins.
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