# MRT reuse in blocked/refined stepping (candidate)

Base `9805383`. Opt-in by `LBMConfig(collision="MRT")`; SRT remains the default and its
kernel statements, including the shifted retained-delta/direct-momentum path, are unchanged
under the alternate compile branch.

## What changes
- `blocks.cu` (`BlockStep`), `step/block_channel.cu` (`BlockChannelStep`) and `refinement.cu`
  (`patch_collide`) gain a compile-time `COLLISION_MODEL == 2` branch that repeats the uniform
  `collide_dispatch` sequence with zero forcing: `compute_rho_u`, omega clamp to
  `[OMEGA_COLLISION_MIN, OMEGA_COLLISION_MAX]`, `collide_mrt_no_force`.
- Kernel and host guards accept SRT or MRT only. TRT, LES, non-FP32 storage, non-double-buffer
  streaming, and shifted dense/mixed refinement remain rejected.
- Transfers, subcycling, BFL, raw exchange ledgers and channel closures are untouched. The
  existing pressure-NEQ outlet selects its MRT branch (unknowns-only NEQ extrapolation) from
  `COLLISION_MODEL`, so MRT channel parity must use uniform MRT as reference.

## Contracts (existing operator, not redefined here)
- Shifted storage uses the existing MRT representation (equilibrium moments from `rho - 1`),
  not the SRT retained-delta/direct-momentum precision. Mass and momentum are conserved
  moments (relaxation rate 0).
- Kinetic rates are the operator's fixed `s_e = s_eps = s_q = 1.2` on every level; the shear
  rate is the existing per-level omega. Under acoustic 2:1 scaling the physical bulk viscosity
  therefore halves on each finer level. No rate rescaling is applied.
- No LES, no external forcing (`d_params` force fields stay zero).
- Shifted Jets stay SRT-only: `body/jets.py` keeps its existing guard. This extension does not
  provide shifted MRT Jets.
- `PeriodicBlockStepper` (uniform blocked stepper) remains SRT-only.

## Evidence status
CPU: `tests/unit/test_refinement_mrt.py` (host guards, source contract). GPU:
`tests/validation/multigrid/test_refinement_mrt_gpu.py` (uniform-MRT channel parity, AMR MRT
smoke) and unchanged SRT GPU tests. Strict parity may be subject to cross-kernel FMA
contraction differences already documented for SRT. Not a DFG/benchmark validation.