sched: require proven ownership at poll/admin/recovery entries (F5)

src/wf_scheduling/ownership.py

@@ -39,6 +39,16 @@ class SchedulerOwnership:
     def lock_path(self) -> Path:
         return self.root / "scheduler.lock"
 
+    @property
+    def held(self) -> bool:
+        """Whether this process holds the lock through this object.
+
+        Only a successful :meth:`acquire` sets this: a second process can
+        never observe ``held`` while another owner holds the OS lock, so
+        entry-point guards can treat it as proof of exclusive ownership.
+        """
+        return self._locked
+
     def acquire(self) -> SchedulerOwnership:
         """Acquire the held lock non-blockingly or raise SecondOwnerError."""
         self.lock_path.parent.mkdir(parents=True, exist_ok=True)