serve-frank-large.sh: only pass --model-draft when SPEC_TYPE contains
'draft-mtp'. ngram-only configs (ngram-mod, ngram-simple, etc.) do not
need a draft model file — loading one wastes VRAM and may error.
bench-spec-decode.py: new benchmark script. Sweeps 4 configs (none,
ngram-mod, draft-mtp, draft-mtp+ngram-mod), generates 16k tokens per
config at temp 0 from a minimal prompt. Uses HTTP API (llama-bench
cannot target --spec-type). Follows benchmarking policy: stop+mask
the production service, bench, unmask+restart, health check.