Add experimental kinematic flow coupling

[marpaia]

Add finite-aspect Jeffery drift for free rods, with normalized midpoint substeps controlling translation and rotation. The angular limit bounds substeps instead of clipping total motion. Fixed cells remain attached; motion is an equivalent-spheroid approximation followed by contact relaxation, not a detachment or cell-resolved hydrodynamic model.

Add a checkpointed backward-Euler option for positive stiff affine transport/reactions. Explicit cell sinks still require an affordable step. Native biological steps now roll back growth, species, grid levels, time, and the solve report when uptake or a solve fails. Controller callbacks and separate mechanics operations remain outside that transaction.

Validation includes finite-aspect shear and rigid-rotation convergence, fixed cells, stiff decay and diffusion, uptake balance in all integration modes, failure rollback/retry, enzyme-amount deposition, and checkpoint continuation. The native suite passes 23/23 on CPU/Metal; CUDA 12.8.1 compilation passes. No NVIDIA runtime or predictive adhesion calibration is claimed.