hive-runtime: TurnStop replaces the subagent daemon's Stopper

TurnStop is the enum over hive-claude's Cancel and the ACP Canceller, with
stop() -> bool (false when no turn was in flight). It signals SIGINT only and
never waits. The subagent daemon tracks a TurnStop per turn, so interrupt has
one stop path; whether the name stays claimed afterwards (claude) or is freed
(ACP) still follows the runtime.

hive-runtime/src/acp/mod.rs

@@ -1029,7 +1029,7 @@ mod tests {
     use hive_claude::{Config, NoopSink, PercentPolicy, Sink};
 
     use super::{AcpError, AcpRuntime, Choice, ReportedCost, SessionChoices, Spend, TurnKind};
-    use crate::{AcpCommand, Error, Runtime};
+    use crate::{AcpCommand, Error, Runtime, TurnStop};
 
     /// An ACP agent in plain `sh`. It answers `initialize`, numbers its
     /// sessions `s1`, `s2`, …, appends every method it is sent to
@@ -1422,6 +1422,24 @@ done
         assert!(!canceller.cancel(), "the turn is over");
     }
 
+    #[tokio::test]
+    async fn a_turn_stop_cancels_only_a_turn_in_flight() {
+        let dir = tempfile::tempdir().unwrap();
+        let runtime = runtime(dir.path(), "silent");
+        let stop = TurnStop::from(runtime.canceller().unwrap());
+        assert!(!stop.stop(), "no turn is in flight yet");
+
+        let config = config(dir.path());
+        let (done, was_in_flight) = tokio::join!(runtime.run(&config, "one", &NoopSink), async {
+            prompt_sent(dir.path()).await;
+            stop.stop()
+        });
+        assert!(was_in_flight);
+        assert!(done.is_ok(), "{done:?}");
+        assert_eq!(count(dir.path(), "session/cancel"), 1);
+        assert!(!stop.stop(), "the turn is over");
+    }
+
     #[tokio::test]
     async fn an_agent_ignoring_a_cancel_is_killed() {
         let dir = tempfile::tempdir().unwrap();