# Compatibility and API changes

Changes that can make a previously accepted setup or call behave differently.
Each entry names the commit, the affected entry points and the observable result.

## 87c3a31: cut-link ownership is validated for every body

`validate_cut_link_ownership` (`body/preprocess/flag_overlay.py`) now runs for
passive bodies as well as Jet/aperture bodies. Every cut link must have an
active fluid receiver (FLUID set, SOLID clear), and each `(receiver, direction)`
population may have only one writer.

Rejected with `ValueError`, before flags, populations or device buffers change:

1. Overlapping or touching passive solids whose cut links write the same
   population ("Boundary requires unique cut-link writers").
2. A body whose cut-link receiver is a non-fluid cell, e.g. a channel wall row
   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 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 (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.