test: cover graceful-leave instant-evict and rejoin-after-eviction edges

Two reconnect-eviction edges were logically handled but had no explicit
assertion:

- A graceful ConnEvent::Left must emit PeerLeft *immediately* (not wait on
  the grace timer). New graceful_leave_evicts_immediately asserts the leave
  lands well inside the grace window.
- After a grace-timer eviction scrubs the peer from seen_connected, a later
  rejoin's initial dial must be a fresh first-dial, not a reconnect that
  arms a new eviction. New rejoin_after_grace_eviction_dials_cleanly covers
  this (distinct from the existing graceful-leave scrub path).

Also reorder the eviction task to scrub timers/seen_connected before
emitting PeerConnectionFailed, so an observer (or a redial racing the
eviction) always sees clean internal state — this also makes the new
rejoin-after-eviction test deterministic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/reconnect_eviction.rs

@@ -68,6 +68,24 @@ async fn evicted_within(
     }
 }
 
+/// Drain UI events until a `PeerLeft` for `peer` arrives, or `within` elapses.
+/// Returns whether the leave fired.
+async fn left_within(
+    ui_rx: &mut mpsc::Receiver<UiEvent>,
+    peer: EndpointId,
+    within: Duration,
+) -> bool {
+    let deadline = tokio::time::Instant::now() + within;
+    loop {
+        match tokio::time::timeout_at(deadline, ui_rx.recv()).await {
+            Ok(Some(UiEvent::PeerLeft { id })) if id == peer => return true,
+            Ok(Some(_)) => continue, // ignore PeerConnecting / PeerConnected
+            Ok(None) => return false, // channel closed
+            Err(_) => return false,   // timed out — no leave
+        }
+    }
+}
+
 const GRACE: Duration = Duration::from_millis(200);
 
 /// The regression: connect, drop, reconnect, then drop again — the second outage
@@ -139,3 +157,48 @@ async fn graceful_leave_lets_a_later_rejoin_dial_cleanly() {
         "a rejoin's initial dial after a graceful leave must not arm eviction"
     );
 }
+
+/// A graceful leave evicts *immediately* — it emits `PeerLeft` right away rather
+/// than leaving the peer "reconnecting" until the grace timer (or the slow gossip
+/// Leave) catches up.
+#[tokio::test]
+async fn graceful_leave_evicts_immediately() {
+    let (ui_tx, mut ui_rx) = mpsc::channel(100);
+    let h = make_handler(ui_tx, make_transport().await, GRACE);
+    let peer = fake_peer();
+
+    h.handle(ConnEvent::Connected(peer)).await;
+    h.handle(ConnEvent::Left(peer)).await;
+
+    // Well inside the grace window: the leave must not be gated on the timer.
+    assert!(
+        left_within(&mut ui_rx, peer, GRACE / 2).await,
+        "a graceful leave must emit PeerLeft immediately, not wait for the grace timer"
+    );
+}
+
+/// After a peer is evicted by the grace timer, its identity is scrubbed from
+/// `seen_connected`, so a later rejoin's initial dial is a fresh first-dial and
+/// must not be mistaken for a reconnect (which would arm a new eviction clock).
+#[tokio::test]
+async fn rejoin_after_grace_eviction_dials_cleanly() {
+    let (ui_tx, mut ui_rx) = mpsc::channel(100);
+    let h = make_handler(ui_tx, make_transport().await, GRACE);
+    let peer = fake_peer();
+
+    // Connect, drop, and let the grace timer fire (evicts + scrubs seen_connected).
+    h.handle(ConnEvent::Connected(peer)).await;
+    h.handle(ConnEvent::Connecting(peer)).await;
+    assert!(
+        evicted_within(&mut ui_rx, peer, GRACE * 4).await,
+        "the first outage should evict once the grace window elapses"
+    );
+
+    // A later rejoin's initial dial: scrubbed on eviction, so this is a first-ever
+    // dial again and must not arm a new eviction timer.
+    h.handle(ConnEvent::Connecting(peer)).await;
+    assert!(
+        !evicted_within(&mut ui_rx, peer, GRACE * 3).await,
+        "an initial dial after a grace eviction must not be treated as a reconnect"
+    );
+}