tools: add llama.cpp comparison baseline + standard benchmark suite

Vendor llama.cpp as a submodule pinned to b9371 and add a one-click
benchmark driver that compares xserv against it on identical workloads:

- setup-llama-cpp.sh: network-optional CUDA build (SM120); convert-to-gguf.sh
  converts the same safetensors to BF16 GGUF for an apples-to-apples baseline.
- tools/bench/: black-box OpenAI-API driver measuring TTFT/TPOT/throughput
  (single-stream + concurrent) and response quality on AIME 2025 + GSM8K.
- fetch_datasets.py pulls datasets to local JSON (GPU host has no network);
  task loaders prefer the local JSON.
- sync-and-build.sh: `bench` subcommand transfers source + datasets to the
  GPU host via tar-over-ssh (no rsync there), builds, and runs the suite.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -7,3 +7,15 @@
 **/*.rs.bk
 .env
 *.npy
+
+# llama.cpp baseline (cloned/submoduled by tools/setup-llama-cpp.sh)
+/third_party/llama.cpp/build/
+/third_party/llama.cpp/models/
+*.gguf
+
+# Benchmark output + fetched datasets (transferred to GPU host, not committed)
+/bench-out/
+/tools/bench/data/
+/tools/bench/__pycache__/
+/tools/bench/**/__pycache__/
+