perf: frank-default-large ubatch 512 (compute graph scales with ubatch; 602100 ctx KV leaves ~16GB, ubatch 2048 OOM)

scripts/serve-llama-frank-large.sh

@@ -36,7 +36,7 @@ fi
 : "${CACHE_TYPE_K:=q8_0}"
 : "${CACHE_TYPE_V:=f16}"
 : "${BATCH_SIZE:=2048}"
-: "${UBATCH_SIZE:=2048}"
+: "${UBATCH_SIZE:=512}"
 : "${THREADS:=8}"
 : "${THREADS_HTTP:=8}"