Use --alias frank-default-large, multi-stream validation script

- Replace --override-kv with --alias for model naming
- Validation script tests 1, 2, 3 parallel streams sequentially
- Model name confirmed: frank-default-large

scripts/serve-llamacpp-dflash.sh

@@ -39,4 +39,5 @@ exec "${SERVER_BIN}" \
     --port "${PORT}" \
     --host "0.0.0.0" \
     --threads 16 \
+    --alias frank-default-large \
     --load-mode none