SA under-relaxed working-variable update

One-equation SA advances the working variable with optional under-relaxation:

\[q \leftarrow (1-c)\,q_0 + c\,(q_0+\Delta q)\]

where (c=1) recovers the full explicit update.

References

• Implementation practice for SA positivity / stability.

• See sa_update_bounds.rst and verification_layers.rst.

Pure API: SaUnderRelaxedUpdate.