feat: add deterministic CPU/GPU benchmark runner slice

.scratch/distributed-gguf-runtime/evidence/DGR-001/README.md

@@ -19,6 +19,21 @@
   - concurrency levels `1` and `4`
   - the required metrics list
   - an explicit stop condition for “no meaningful speed or fit benefit”
+- Adds a deterministic stub benchmark report so the contract now has an executable report shape end to end.
+
+## Recent benchmark runner slice
+
+The runner currently uses a deterministic stub backend to exercise the comparison matrix without downloading a model. It emits:
+
+- `.scratch/distributed-gguf-runtime/evidence/DGR-001/performance-contract.json`
+- `.scratch/distributed-gguf-runtime/evidence/DGR-001/stub-benchmark-report.json`
+
+The report includes per-device comparisons for:
+
+- `transformers-safetensors-cpu` vs `llama-cpp-gguf-cpu`
+- `transformers-safetensors-gpu` vs `llama-cpp-gguf-gpu`
+
+and records the memory metric (`rss_bytes` on CPU, `vram_bytes` on GPU), decode speedup, artifact ratio, and output drift.
 
 ## Exact commands and real results