feat: add 21 interview notes from 7 XHS posts

Sources:
- KV Cache夺命追问破局指南下 (5 questions: 量化/Offloading/稀疏注意力/可观测/OOM处置)
- 前端AI项目必问：为啥不能只用向量检索 (6 questions: 混合检索/RRF/Reranker/RAG优化/多轮对话/Embedding选型)
- 明星官宣离婚热点缓存 (1 question: 千万级流量缓存架构)
- JAVA大厂Nacos踩坑 (1 comprehensive note: 实例类型/一致性协议/配置刷新/Eureka对比)
- Java后端转AI Agent面试 (4 questions: 记忆机制/幻觉防护/工具调度/任务排序)
- 快手AI大模型开发面经 (4 questions: LoRA人设微调/RAG幻觉治理/异步批量架构/FC准确率+长对话压缩)

Total: 21 new notes (19 AI + 1 system-design + 1 java)
All passed three-pass review (YAML structure + content + quality)