feat(lib): cap the combined download speed

`Client::set_download_speed_limit(bytes_per_second)` puts one ceiling on
every download at once. Zero lifts it and is the default. It can be
changed at any time, and a change reaches transfers already running
within one 100 ms tick.

The limiter works the way Soulseek.NET's does, which slskd uses for its
own download limit: one token bucket shared by all downloads, refilled
to a tenth of a second's worth every 100 ms, handing out partial grants
and taking back what a short read or a timeout did not use. A download
is paced by reading less from the socket. The unread bytes stay in the
kernel's receive buffer and TCP flow control slows the uploader; no
protocol message is involved, as in Nicotine+ too.

With no cap set, each read costs one atomic load. A same-hour stress
run scored develop 88.98/88.98 and this branch 89.00/88.99/88.97, at
about 133 MiB/s both.

An upload ceiling is left out, as before.

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

@@ -14,6 +14,12 @@ impl Client {
             .unwrap_or_default()
     }
 
+    pub fn set_download_speed_limit(&self, bytes_per_second: u64) {
+        if let Ok(ctx) = self.context.read_safe() {
+            ctx.download_speed_limit.set(bytes_per_second);
+        }
+    }
+
     #[must_use]
     pub fn pause_download(&self, username: &str, filename: &str) -> bool {
         match self.context.write_safe() {