refinement: document that the free-memory preflight is a lower bound only

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CelerisLab/lbm/refinement_stepper.py

@@ -373,7 +373,11 @@ class FixedPatchStepper:
                 raise
 
     def _planned_bytes(self):
-        """Both G buffers, halo pairs and boundary columns of every level (transfer tables excluded)."""
+        """Both G buffers, halo pairs and boundary columns of every level (transfer tables excluded).
+
+        A fail-fast lower bound only: passing it cannot guarantee that later
+        allocations succeed (other consumers, fragmentation, races).
+        """
         total = 0
         for index, layout in enumerate(self._layouts):
             cells = int(np.prod(layout.blocked_shape if layout is not None