frank-small: revert to bf16 KV — the only dtype that boots Gemma4

int8_per_token_head: page-size unification fails (scale tensor breaks
the 256/512 head-dim ratio). turboquant_k8v4: backend validation fails
(no mm-prefix attention, which Gemma4's vision/audio path requires).
fp8: hardware-dead on sm_86. bf16 it is; history documented in
frank-small.env, seam pins auto + no backend pin.

scripts/serve-frank-small.sh

@@ -44,7 +44,6 @@ fi
 : "${MAX_NUM_SEQS:=16}"
 : "${MAX_NUM_BATCHED_TOKENS:=16384}"
 : "${SPEC_NUM_TOKENS:=4}"
-: "${ATTENTION_BACKEND:=TURBOQUANT}"
 : "${ENFORCE_EAGER:=0}"
 
 # Fail loudly rather than half-loading a model into VRAM: a missing file here
@@ -67,7 +66,6 @@ VLLM_FLAGS=(
 --kv-cache-dtype "$KV_CACHE_DTYPE"
 --max-num-seqs "$MAX_NUM_SEQS"
 --max-num-batched-tokens "$MAX_NUM_BATCHED_TOKENS"
---attention-backend "$ATTENTION_BACKEND"
 --speculative-config "$SPEC_CONFIG"
 --reasoning-parser gemma4
 --enable-auto-tool-choice