fix!: make concurrent invocations work instead of silently seeing nothing (#22)

Running several one-shot commands at once returned "no results" for queries that
had plenty. Four defects, each of which made a dead session look like an empty
network:

- The peer listener bound its port inside a detached thread and panicked on
  EADDRINUSE, unwinding that thread alone. The process carried on with no
  listener, so no peer could deliver a search response. `Listen::bind` now takes
  the socket before login and falls back to an ephemeral port when the
  configured one is held.
- SetWaitPort advertised the configured port whether or not it was bound, so
  peers dialled a sibling process that discarded responses to tickets it never
  issued. The port the listener really holds is what gets advertised, mapped
  through UPnP, and reported by `whoami` and `serve`.
- A zero-byte read is EOF on a TCP socket, but MessageReader reported it as
  success, so a server that closed cleanly left the actor polling a dead socket.
  It is now an error, and Relogged (code 41) has a handler. The peer actor
  classifies that error: a control connection that was established and then
  closed is a clean disconnect, because peers drop idle sockets while the bytes
  move over their own connection, and reporting an error there fails every
  download queued with that peer including the one streaming. A connection that
  never established keeps the error path, which is what the server-brokered
  fallback reads to know a peer is unreachable.
- A session the server cut off - usually because another process logged in under
  the same name - exited 4, documented as "it worked and found nothing". Callers
  concluded the file was unavailable. Sessions now record why they ended and
  those verdicts become exit 7.

The server allows one session per account, so concurrency still needs a distinct
--username per run; SKILL.md documents that. The listener port needs no such
care any more.

Six e2e tests against soulfind cover it, each of which fails without the fix: a
taken listen port, a displaced session, three clients searching at once under
one configured port, the two CLI equivalents, and a peer that hangs up its
control connection while a transfer is still to come.

BREAKING CHANGE: `Listen::start` is replaced by `Listen::bind` and
`Listen::serve`, so the caller owns the socket and can read the port it actually
bound. `Client::listen_port` and `Client::session_loss` are new.

soulseek-rs-lib/src/types.rs

@@ -399,3 +399,62 @@ pub struct UploadInfo {
     /// download does. Zero unless the upload is in progress.
     pub speed_bytes_per_sec: f64,
 }
+
+/// Why a server session ended before the client was done with it.
+///
+/// A session that ends stops seeing the network entirely, which is not the
+/// same thing as the network having nothing to show — telling the two apart is
+/// the difference between "retry" and "give up".
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum SessionLoss {
+    /// Another login claimed this username. The server keeps one session per
+    /// name, so the older one is cut off.
+    Displaced = 1,
+    /// The connection to the server dropped.
+    Disconnected = 2,
+}
+
+impl std::fmt::Display for SessionLoss {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Displaced => f.write_str(
+                "another login took over this username; the server allows one \
+                 session per account",
+            ),
+            Self::Disconnected => {
+                f.write_str("the connection to the server dropped")
+            }
+        }
+    }
+}
+
+/// A lock-free view, shared with the server actor, of whether the session is
+/// still alive.
+///
+/// The first loss recorded wins: a displaced session also drops its socket a
+/// moment later, and "displaced" is the reason worth reporting.
+#[derive(Debug, Clone, Default)]
+pub struct SessionWatch(std::sync::Arc<std::sync::atomic::AtomicU8>);
+
+impl SessionWatch {
+    const LIVE: u8 = 0;
+
+    pub fn record(&self, loss: SessionLoss) {
+        let _ = self.0.compare_exchange(
+            Self::LIVE,
+            loss as u8,
+            std::sync::atomic::Ordering::Relaxed,
+            std::sync::atomic::Ordering::Relaxed,
+        );
+    }
+
+    #[must_use]
+    pub fn loss(&self) -> Option<SessionLoss> {
+        match self.0.load(std::sync::atomic::Ordering::Relaxed) {
+            1 => Some(SessionLoss::Displaced),
+            2 => Some(SessionLoss::Disconnected),
+            _ => None,
+        }
+    }
+}