Move the Vec<Download> and the download lifecycle methods (add, remove,
lookup, status update, pause, resume, remove-if-queued) into a new
download_store module. ClientContext keeps thin delegating shims so
peer/listen, peer/download_peer and the existing client tests carry on
unchanged. Client::pause_download / resume_download / remove_queued_download
now delegate one-liners. Client::process_failed_uploads keeps the
read-then-write lock dance but the inner filtering is the new
collect_failed_tokens helper.
Adds dedicated unit tests for the store's behavior (add/get/remove,
pause+resume, queued-only removal, failed-token collection) so the
state-machine logic is exercised independently of Client.
Add SoulseekRs::LockPoisoned variant and a small RwLockExt/MutexExt
helper module exposing read_safe/write_safe/lock_safe that map
PoisonError to that variant. Walk every non-test lock acquisition
across client, peer/server actors, peer registry, listener and the
thread pool: propagate via ? where the function returns Result, log
and degrade to a sensible default (return early, false, None, empty)
where it does not. Client::connect now returns Result so the CLI can
surface the failure instead of panicking.
michel