docs: add Pass 0b cost benchmark — v3 vs v4 vs backfill vs Mac Mini

Documents all cost optimization attempts:
- v4 Haiku direct: $33/10k (RECOMMENDED)
- v3 + Haiku backfill: $31/10k (not worth the complexity)
- v3 + Mac Mini qwen3: $25/10k + 77h wait (too slow, weaker quality)
- Token analysis, quality comparison, lessons learned

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mkdocs.yml

@@ -123,3 +123,4 @@ nav:
     - QA Control Quality: development/qa-control-quality.md
     - RAG Pipeline Lessons Learned: development/rag-pipeline-lessons-learned.md
     - RAG Pipeline Benchmark: development/rag-pipeline-benchmark.md
+    - Pass 0b Kosten-Benchmark: development/pass0b-cost-benchmark.md