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>
