fix(distributed): a server with no parent figures leaves our cap alone

The speed-derived child limit read a missing ratio as a refusal, so on any
server that never announces ParentSpeedRatio — soulfind among them — the
client carried no children at all, however it was configured. A ratio we
were never told is not a decision: the server's figures set the limit only
when it has given them, and otherwise the configured cap stands, which is
how slskd runs throughout. A speed below the announced minimum still means
none, which is the safety property the rule exists for.

Also stops repeating AcceptChildren: it is a standing state on the server,
so it goes out when the answer changes rather than on every message that
happens to prompt a recount.

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

@@ -466,6 +466,9 @@ pub struct ClientContext {
     leaf: distributed::Leaf,
     /// The children hanging from us, when the client serves any.
     pub(crate) children: children::Children,
+    /// The last `AcceptChildren` answer we gave the server, so the same
+    /// answer is not repeated: it is a standing state, not a heartbeat.
+    announced_accept_children: Option<bool>,
     /// The upload speed the server records for us, once it has told us. The
     /// child limit is derived from it.
     own_average_speed: Option<u32>,
@@ -642,6 +645,7 @@ impl ClientContext {
             operations: None,
             leaf: distributed::Leaf::new(""),
             children: children::Children::default(),
+            announced_accept_children: None,
             own_average_speed: None,
             parent_min_speed: 0,
             parent_speed_ratio: 0,
@@ -829,6 +833,16 @@ impl ClientContext {
         self.own_interests.clone()
     }
 
+    /// Take the `AcceptChildren` answer to send, or `None` when the server
+    /// already has this one.
+    pub fn accept_children_change(&mut self) -> Option<bool> {
+        let has_room = self.children.has_room();
+        (self.announced_accept_children != Some(has_room)).then(|| {
+            self.announced_accept_children = Some(has_room);
+            has_room
+        })
+    }
+
     /// The upload speed the server records for us, if it has said.
     #[must_use]
     pub const fn own_average_speed(&self) -> Option<u32> {