#310: log when a reap is skipped because the record changed

The compare-and-release declines silently. This race is unobservable by
construction, so a reaper that quietly stops reaping is the hardest kind of
behaviour to diagnose later. One debug line names the pane and the likely
cause.

fleetd/src/main/java/dev/ltms/fleet/session/SessionManager.java

@@ -294,6 +294,11 @@ public final class SessionManager implements TurnListener {
      */
     private boolean releaseIfCurrent(MemberSession expected, ReleaseCause cause) {
         if (!registry.remove(expected.paneId(), expected)) {
+            // A lifecycle transition replaced the record between the caller's check and this remove.
+            // Log it: this race is by definition unobservable otherwise, and a reaper that silently
+            // declines to reap is the hardest kind of behaviour to diagnose after the fact.
+            log.debug("skipping reap of pane={}: its registry record changed after the idle check "
+                    + "(most likely a delivery made it BUSY)", expected.paneId());
             return false;
         }
         releaseRemoved(expected.paneId(), expected, handles.remove(expected.paneId()), cause);