Opt-in third NLI inference backend (ARBORIST_NLI_BACKEND=tinygrad) in
qa/nli/shadow.py, parallel to torch/onnx-int8, behind the #000049
cage: shadow-only, never an audit_mode input, never auto-preempts the
proven path (guarded so it cannot regress torch/onnx). Loads the ONNX
export through tinygrad's frontend wrapped to the existing
model(**enc).logits contract so _nli_batch is byte-unchanged.
bench/scripts/nli_backend_ab.py + make bench-nli-backends: deterministic
A/B, gate is numerical agreement with the torch reference first,
latency second (a divergent engine = a different shadow signal = a
different nli_policy_hash). Instrument is honest — reports a requested
backend as unavailable rather than relabelling a fallback's numbers.
First CPU-smoke run already quantified that the deployed §7 #22 int8
export diverges Δmax≈0.42 from torch — the immunity property made
measurable, not a defect. Real tinygrad numbers pending a producer-box
run (tinygrad not an arborist dep; frontend op-coverage for the large
MNLI checkpoints unverified by design).
docs/onnx-vendor-capture-immunity.md: why the model-in-proof-path cage
makes the inference engine an interchangeable sidecar, never a trust
dependency — public-domain positioning capital. Indexed in CLAUDE.md.
Full suite 2498 passed (identical to baseline); 24/24 NLI tests green.
russell@unturf.com
a4f3e126f7