Add safe optimization flags to gemma4 + qwen36 (fastsafetensors, prefix-caching, fp8 kv)

Aligned with sibling recipes in eugr/spark-vllm-docker. Applies on next swap to each model.
First real swap gemma4 -> qwen36 succeeded in 5:30 with --moe_backend=flashinfer_cutlass.

runbook.md

@@ -2,6 +2,20 @@
 
 Operating notes for running and maintaining the cluster via spark-control.
 
+## Recent successful swaps
+
+- **2026-05-12 — gemma4 → qwen36** via `POST /api/swap` from laptop dev server. ~5:30 to "Application startup complete." Inference works (`/v1/chat/completions` returns reasoning content via `reasoning` field). `--moe_backend=flashinfer_cutlass` confirmed valid by vLLM (logged "Using 'FLASHINFER_CUTLASS' NvFp4 MoE backend").
+
+## Optimization flags (added 2026-05-12)
+
+Aligned `gemma4` and `qwen36` `vllm_args` with the project's sibling recipes (`qwen3.5-35b-a3b-fp8.yaml`, `gemma4-26b-a4b.yaml`):
+
+- `--load-format=fastsafetensors` — faster cold-start weight load.
+- `--enable-prefix-caching` — reuse cached prefix tokens (e.g. system prompt) across requests.
+- `--kv-cache-dtype=fp8` — store KV cache in 8-bit FP; halves memory used per active context.
+
+These take effect on the **next swap to that model**. If a swap fails after this change with errors mentioning fastsafetensors/prefix-caching/fp8, revert the entry in `models.yaml` and retry.
+
 ## Day-to-day
 
 - The UI lives at `http://<your-start9>.local:9999` once the StartOS package is installed and configured.