frank-small: pin VLLM_ATTENTION_BACKEND=TURBOQUANT

Gemma4's config hook force-selects TRITON_ATTN for the mixed 256/512
head dims whenever no backend is selected, and TRITON_ATTN rejects the
turboquant KV dtype (measured: 'kv_cache_dtype not supported', crash
loop). The TURBOQUANT backend accepts any head size and the
turboquant_* dtypes, and the Gemma4 MTP speculator carries the
target's selected backend through to the draft layers.

tests/frank_small_unit.bats

@@ -98,7 +98,7 @@ QUADLET_LARGE="$REPO_ROOT/dankclaw/frank-large/quadlet/frank-large.container"
     ! grep -q 'HALOGEN_VISION_TOWER' "$QUADLET_LARGE"
 }
 
-@test "frank-small serves the full window with turboquant k8v4 KV on a dedicated card" {
+@test "frank-small serves the full window with turboquant KV on a dedicated card" {
     local maxlen kvdtype util seqs
     maxlen="$(sed -nE 's/^MAX_MODEL_LEN=([0-9]+)$/\1/p' "$ENV_FILE")"
     kvdtype="$(sed -nE 's/^KV_CACHE_DTYPE=(.+)$/\1/p' "$ENV_FILE")"
@@ -113,6 +113,10 @@ QUADLET_LARGE="$REPO_ROOT/dankclaw/frank-large/quadlet/frank-large.container"
     # (head_dim 256) and full-attention (head_dim 512) layers, and fp8 is
     # hardware-dead on sm_86 anyway.
     [ "$kvdtype" = "turboquant_k8v4" ]
+    # The TURBOQUANT backend must be pinned explicitly: Gemma4's config
+    # hook force-selects TRITON_ATTN (mixed 256/512 head dims) whenever no
+    # backend is selected, and TRITON_ATTN rejects the turboquant dtype.
+    grep -q '^VLLM_ATTENTION_BACKEND=TURBOQUANT$' "$ENV_FILE"
     # The 3090 is dedicated to this service, so the engine may take most of
     # it; the KV pool, not the seq ceiling, caps full-context concurrency.
     [ "$util" = "0.95" ]