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

PROTOCOL.md

@@ -275,10 +275,14 @@ IO; only overlapping opens and rename/removal wait for one another.
 Watch the narrowest useful path: delivery is not rate-limited, so watching a
 busy parent can cause a rescan for every write.
 
-The watch ends with its caller; there is no cancellation API or polling/retry
-loop. SDK callers should poll the watch future alongside their handler's work
-and drop it on return, not keep it in a background task that prevents an idle
-process from exiting. Dropping the future does not cancel the host watch.
+The host-side watch ends only when its caller process exits, not on a change
+notification; there is no cancellation API or polling/retry loop. SDK callers
+should poll the watch future alongside their handler's work and drop it on
+return, not keep it in a background task that prevents an idle process from
+exiting. Dropping the future removes the SDK's pending entry and callback, so
+late notifications and responses are ignored; it does not cancel the host
+watch. A long-lived service keeps one watch per purpose for its lifetime,
+rather than repeatedly opening and dropping watches.
 
 ## 5. Endpoints