ARB-APR-3: parity receipt for the decide lane's cell on the agent's host (Qwen3.5-9B-Q4_K_M @ lambda-labs) #3574

ARB-APR-001 row 3, aprender-owned. rust-agentic-harness.md §3.4: a lane on a backend that has not passed parity is not a reviewer — so the apr decide lane needs a committed parity receipt for the cell it runs.

Produced 2026-09-20 rather than waiting. An earlier note here held this row until the agent host's declared apr carries CUDA (ARB-APR-2a, blocked on forjar's cargo provider having no features: field). That was the wrong call: the CLI works today — apr run answers 20/20 cells across 0.8B–27B on two GPU hosts — and holding the receipt because a preferred cell is not declared yet ships nothing while the lane runs unreceipted.

Measurement, published 0.68.2 built from crates.io with --features cuda, on the agent host (noah-Lambda-Vector, RTX 4090, sm_89, x86_64):

apr parity ~/models/Qwen3.5-9B-Q4_K_M.gguf --prompt "<68-token corpus>" --json
→ 68 positions, parity: true, passed 68 / failed 0
→ min cosine 0.9987 at position 44   (threshold for qwen3.5-9b: 0.99)
→ judged: scripts/check_model_parity.sh --judge … --model qwen3.5-9b → PASS

What this receipt does and does not certify. It certifies the (model, quant, host) cell: Qwen3.5-9B-Q4_K_M on this host passes CPU-vs-CUDA parity at the shipped threshold. The decide lane's binary on this host is currently the default-features build (default = ["cli"], no CUDA), so the lane runs the CPU arm of a cell whose CPU and CUDA arms are now measured to agree at 0.9987. That is a loud, receipted state — the doctrine allows it; what it does not allow is silence.

When ARB-APR-2a lands and the declared resource carries --features cuda, the same command re-run on the CUDA arm replaces this record rather than supplementing it, and the backend named in the run receipt must match the declared backend (ARB-APR-2's own RED).

Remaining for done_when: the infra decide config must reference this receipt, and decide must refuse apr cell unreceipted when the reference is missing. That half is infra's.

Refs #3080, #3571, ARB-APR-001 row 3