feat: add 11 interview questions from PDD/QPS100x/12306 XHS posts

- PDD Java面经(9题): LRU并发安全, ThreadLocal内存泄漏, synchronized锁升级,
  死锁排查, OOM排查, OOM无Dump, G1 MixedGC, binlog/redo/undo, 2PC崩溃恢复
- QPS突增100倍(1题): 5阶段应急方案
- 12306抢票系统设计(1题): 分层削峰百万并发

All questions include: Feynman method + First Principles + ASCII diagrams +
code examples + Socratic follow-up questions + 面试加分点

Source: 小红书 XHS posts (拼多多面经/QPS100倍/12306抢票)