# Matched MRT DFG pilot preparation

Preparation only: this test-only extension reuses the existing runner and existing MRT
operator. GPU admission awaits reviewed MRT capability results; no pilot/full execution
is implied by CPU preflight. No production, historical SRT manifest, reference data,
periodic gate, geometry, initialization or observation algorithm is changed here.

## Selection and provenance

Use `--collision MRT` for **both** uniform D80 and refined root D40/fine D80. There are
no independently selectable collision models per case. Default remains SRT.

MRT verifies `data/refined_dfg/source_manifest_mrt.json`, pinned to reviewed numerical
source snapshot `3ea131e92c057843034f11a1c574301a2430dfce`, including the shared geometry,
pressure/cycle helpers and official reference. Test-only commits may change HEAD without
changing this numerical source pin. Plan, READY, case summaries and comparison separately
record `checkout_revision` and `checkout_dirty` using existing source-provenance helper;
the runner hash remains separate. Collision is recorded in every receipt, and full-mode
validation rejects missing or different collision labels in plan, either summary or either
READY receipt, in addition to existing source/runner/health/time checks.

The historical SRT manifest remains byte-for-byte unchanged. SRT on the MRT candidate
checkout deliberately fails that source pin; use a matching historical checkout via
`--source` for SRT. No fallback or automatic repinning is permitted. Historical SRT
failure/diagnostic evidence remains attached to its original source and runner hashes;
it cannot authorize an MRT full run. The existing failed-observation step-reporting
undercount is not changed in this narrowly scoped selection patch.

## Frozen physical comparison

All settings remain those documented in `refined_dfg.md`: U_lattice=.01, physical mean
U=1 m/s, D=.1 m, shear ν=.001 m²/s, fixed physical domain/body/wall planes, patch
(40,24,320,140), shifted FP32 double buffers, no LES/forcing/Jets, finest dx=.00125 m,
finest dt=.0000125 s, unchanged half-cell frame conversion and axis-specific phase.
Force interval accumulation, physical leaf mass, pressure interpolation, flux sampling,
safety thresholds, t*=5 pilot and t*=300 cap are unchanged. Existing accuracy/periodicity
criteria are imported unchanged. Both MRT cases are fresh runs; old MRT or SRT results
are not substituted as controls.

Both cases use the **existing MRT pressure-NEQ branch**: only unknown directions q2/6/8
are extrapolated. This differs from SRT's all-nine .5-damped branch; the retained
`srt_neq_damp=.5` configuration value is inactive under MRT. Backflow clamp remains off.
The outlet is still noncanonical relative to exact DFG do-nothing conditions.

The reused MRT operator keeps fixed kinetic rates `s_e=s_eps=s_q=1.2` on all levels;
only the shear rate follows existing per-level viscosity. Physical bulk relaxation
therefore differs between coarse and fine levels. Transfer operators and subcycling
are unchanged. This is the candidate's declared behavior, not a new fully mode-scaled
MRT transfer scheme. Do not tune rates, the patch or gates after seeing outcomes.
Capability smoke/parity gates do not establish DFG accuracy or speedup.

## CPU and admission commands

From this checkout:

```sh
python tests/unit/test_refined_dfg.py
python tests/validation/run_refined_dfg.py --collision MRT --mode plan --output /tmp/new-mrt-dfg-plan
```

After root verifies the candidate MRT root/parity/refinement capability results and admits
an idle device, the paired pilot may use the existing bounds and external watchdog:

```sh
CUDA_VISIBLE_DEVICES=<admitted-device> timeout --signal=TERM --kill-after=30s <hard-budget>s python tests/validation/run_refined_dfg.py --collision MRT --mode pilot --gpu-gates-approved --max-wall-seconds <soft-budget> --output <new-mrt-pilot-directory>
```

The flag is caller attestation, not evidence. An unhealthy first case prevents the second.
Existing strict pilot receipt validation and calibrated wall-time controls remain; full
mode is not being admitted as part of this preparation. Root must review both healthy
pilot outcomes and separately decide any full-run admission.