Jets: one jet may drive links of only one body

surface_link_velocity (the shared host check used by CurvedLinkSoA,
ObjectManager sync, prepare_fine_boundary and FixedPatchStepper) now
rejects a jet whose selected cut links carry more than one body id, at
any amplitude, before uploads. Identity is the scene-wide body id of the
physical cut-link list, so one body spanning storage blocks stays valid.
No body selector; empty-selection semantics unchanged.

Tests: two bodies under one jet rejected (amplitude 2e-3 and 0) on the
Simulation/manager path and on the fine path; separate jets on separate
bodies accepted on both; one fine body crossing 2x2 block seams accepted.

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

src/CelerisLab/body/jets.py

@@ -6,7 +6,8 @@ packing), over every link in the scene; it carries no body association.
 Per-link velocity = amplitude * profile(s) * field(hit), where ``s`` in [-1, 1]
 is the region's lateral coordinate. Velocities are lattice units of the level
 whose links are evaluated; callers convert frames/units before evaluation.
-One physical link (receiver, direction) may be driven by at most one jet.
+One physical link (receiver, direction) may be driven by at most one jet, and
+one jet may drive the links of only one body (checked at assignment, any amplitude).
 
 Edge rule: the lateral bound |s| < 1 is exclusive (the profile vanishes
 there for parabolic/quartic); Sector r_min/r_max and Tube 0 <= along <= length
@@ -222,6 +223,11 @@ def surface_link_velocity(objects, cfg, flags, fluid_idx, dir, body_id, rx, ry,
             raise ValueError("Jets require D2Q9/FP32/unshifted/double_buffer")
         shape, jet_id = jet_link_shapes(jets, hits)
         driven = jet_id >= 0
+        for j in range(len(jets)):  # scene-wide body identity, independent of storage blocks
+            bodies = np.unique(body_id[jet_id == j])
+            if len(bodies) > 1:
+                raise ValueError(f"Jet {j} selects cut links of bodies {bodies.tolist()}; "
+                                 "one jet may drive only one body")
         owners = {obj.obj_id: obj for obj in objects}
         for bid in np.unique(body_id[driven]):
             state = vars(owners[int(bid)].state)