fix: survive busy sessions without exhausting file descriptors

The peer listener died with "Too many open files (os error 24)" on any
long-lived non-daemon session: only the daemon raised RLIMIT_NOFILE, and
peer sockets were unbounded — no idle reaping, no deadlines on accepted
transfer streams, and one peer could hold two connections under split
registry identities.

- raise the open-file limit at the top of run(), for every entry point,
  and warn from Client::connect when the soft limit is below 1024
- reap peer control connections idle for five minutes; a clean
  disconnect no longer discards the peer's queued uploads
- give listener-supplied download streams and upload serving the same
  read/write deadlines dialled sockets already had
- register inbound peers under their plain username so the registry
  holds one connection per peer, and announce them like pierced ones
- hold the registry lock across actor spawn + insert and surface
  thread-spawn failure, so an instantly-dead peer cannot leave a
  zombie entry

soulseek-rs-lib/src/client/uploads.rs

@@ -5,14 +5,6 @@ use super::{
 use crate::types::UploadStatus;
 use std::sync::atomic::{AtomicBool, AtomicU64};
 
-/// The peer behind a registry key, which may carry a `:direct` suffix the
-/// server never sees.
-pub fn downloader_of(requester_key: &str) -> &str {
-    requester_key
-        .strip_suffix(":direct")
-        .unwrap_or(requester_key)
-}
-
 impl Client {
     /// Whether the server listed `username` as privileged. Privileged peers are
     /// served before everyone else.