Clean up SDK pending calls and notification callbacks on future drop

Preserve late lock-grant release separately, and drop callback captures
outside transport state borrows. Document process-lifetime host watches.

Card: SAU-391

sdk/tests/plugins/probe/src/main.rs

@@ -98,6 +98,36 @@ impl Probe {
         .await
     }
 
+    /// Drop one watch while another still delivers, in a process that remains alive.
+    async fn drop_watch(&self) -> Result<Vec<String>> {
+        use std::{cell::RefCell, future::IntoFuture, rc::Rc, task::Poll};
+        let events = Rc::new(RefCell::new(Vec::new()));
+        let captured = events.clone();
+        let mut dropped = call("core.watch", json!({"path":"/dropped"}))
+            .on_notify(move |_, _| captured.borrow_mut().push("dropped".to_owned()))
+            .into_future();
+        std::future::poll_fn(|cx| {
+            assert!(dropped.as_mut().poll(cx).is_pending());
+            Poll::Ready(())
+        })
+        .await;
+        drop(dropped);
+        let captured = events.clone();
+        let mut kept = call("core.watch", json!({"path":"/kept"}))
+            .on_notify(move |name, _| captured.borrow_mut().push(name.to_owned()))
+            .into_future();
+        std::future::poll_fn(|cx| {
+            assert!(kept.as_mut().poll(cx).is_pending());
+            Poll::Ready(())
+        })
+        .await;
+        // The harness sends late traffic and kept-watch events before answering.
+        call("other.barrier", json!({})).await?;
+        drop(kept);
+        let result = events.borrow().clone();
+        Ok(result)
+    }
+
     /// Publish a control endpoint until EOF.
     async fn serve(&self) -> Result<()> {
         sausage_plugin::listen("/endpoint", std::rc::Rc::new(Control)).await