docs: Phase 18 pipeline parallelism — design + benchmark results

docs/18-pipeline-parallelism.md: PP design (layer split, NCCL P2P,
per-stage KV, engine/threading model).
docs/benchmarks/pp-sweep.md: measured on dash5 (8x RTX 5090, Qwen3-8B
BF16) — single-stream latency + per-GPU VRAM (~1/N), byte-exact
correctness (single x2 vs pp4 x2 control), and the full AIME-30 +
GSM8K-30 quality matrix (xserv & llama.cpp PP=1/2/4): GSM8K 29/30 in
every cell, TPOT flat across PP.
README: multi-card (TP/PP) section + roadmap to Phase 18.
gitignore: /.claude/ runtime state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -13,6 +13,9 @@
 /third_party/llama.cpp/models/
 *.gguf
 
+# Claude Code runtime state
+/.claude/
+
 # Benchmark output + fetched datasets (transferred to GPU host, not committed)
 /bench-out/
 /tools/bench/data/