fix(vllm): right-size granite util 0.34->0.18 + max-len ->65536 (free ~10.5GB GPU1)

Operator-directed 2026-07-16: free ~10GB on ana-ml2 GPU1 to relocate a GPU0
model onto GPU1. granite-4.1-8b (fleet summarizer) was over-provisioned at
util 0.34 / max-model-len 131072 with a flat 0.0% KV usage.

Set GRANITE_GPU_MEM_UTIL 0.34 -> 0.18 and GRANITE_MAX_MODEL_LEN 131072 -> 65536
on the live /opt/docker/compose/vllm/.env (backup .env.bak-pre-granite-rightsize-
20260716), recreated vllm-granite ONLY (shared stack). Result: GPU1 62,641 ->
51,897 MiB used (~10.5GB freed, ~45GB free now); KV 6.45 GiB / 84,528 tok /
1.29x concurrency @ 65536; summarizer verified healthy.

The util drop required the max-len drop: on this shared card the effective KV
slope is ~950 MiB per 0.01 util, and vLLM refuses to start unless the KV pool
holds >= 1x max-model-len -- util 0.15 undershot (est max-len 47184 < 65536,
crash-loop, ~2-3 min summarizer outage) before 0.18 landed. 65536 is granite's
precedented summarizer ctx; a summarizer doesn't need 131072.

.env.example updated to the new util (max-len was already 65536 in the template;
live had drifted to 131072). persistent-memory.md updated (parked item closed).

stacks/vllm/.env.example

@@ -107,4 +107,10 @@ GRANITE_KV_CACHE_DTYPE=fp8
 # qwen36-vl 0.46 + granite 0.24 + reward 0.10 + embed/rerank 0.03 ≈ 0.90 total,
 # ~7.5 GB headroom (the OOM buffer; held under 20-concurrent load test). granite
 # gets a 169K-token KV pool = 2.58x concurrency @ 64K. Bring qwen36 up LAST.
-GRANITE_GPU_MEM_UTIL=0.24
+# 0.18 — RIGHT-SIZED 2026-07-16 (was 0.34 live; qwen36-vl no longer a GPU-1 tenant,
+# image-bench evicted) to free ~10.5 GB for relocating a GPU0 model onto GPU1. KV
+# 6.45 GiB = 84,528 tokens = 1.29x concurrency @ 65536 (summarizer = short parallel
+# calls; ample). Effective slope on this shared card ≈ 950 MiB KV per 0.01 util, and
+# KV must hold >= 1x max-model-len — util 0.15 undershot (crash: est max-len 47184 <
+# 65536), 0.18 lands the target cleanly.
+GRANITE_GPU_MEM_UTIL=0.18