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/mod.rs

@@ -248,7 +248,7 @@ pub enum ClientOperation {
         query: String,
     },
     /// A peer queued one of our shared files; `requester_key` is the registry
-    /// key of the peer actor (may carry a `:direct` suffix).
+    /// key of the peer actor — the peer's username.
     QueueUpload {
         requester_key: String,
         filename: String,