docs: checkpoint save/load scope and read_surface_exchange wording

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

docs/compatibility.md

@@ -18,15 +18,18 @@ Rejected with `ValueError`, before flags, populations or device buffers change:
    or the inlet/outlet column ("Cut links must have active fluid receivers").
 
 Both were previously accepted silently for passive bodies; the interpolated
-boundary then had undefined results. The check runs in
-`Simulation.initialize`, runtime body sync (`sync_bodies`) and checkpoint load
-(`common/checkpoint.py`). A checkpoint whose saved geometry violates either rule
-no longer restores; rebuild the geometry with separated bodies.
+boundary then had undefined results. The rules apply to the cut links of all
+bodies together, so a passive body overlapping a Jet body is rejected as well.
+The check runs in `Simulation.initialize`, runtime body sync (`sync_bodies`),
+checkpoint save and checkpoint load (`common/checkpoint.py`). Load validates
+before restoring anything, so a checkpoint whose saved geometry violates either
+rule no longer restores; rebuild the geometry with separated bodies.
 
 ## 87c3a31: `Simulation.read_surface_exchange` accepts Jet-driven bodies
 
 `read_surface_exchange(id)` previously required `SurfaceActuation` on the body.
 It now also accepts bodies that own Jet aperture links, including Jets with zero
-or negative amplitude. Bodies without any aperture still raise
+or negative amplitude (the aperture test is geometric). Bodies with neither
+`SurfaceActuation` nor aperture links still raise
 `ValueError("Body has no surface actuation")`. The value is unchanged: the latest
 step's wall/aperture exchange, not total nozzle force.