docs: add backend/13-kotlin-style.md and index it

Kotlin syntax & coding constraints for the backend service: treat Kotlin as Java
syntax sugar, not as a new paradigm. Bans !! / lateinit / coroutines / RxJava /
Reactor, data-class entities, and XxxUtils|Util|Helper|Common classes — each rule
states the concrete failure it prevents.

Also records the Gradle two-sided version constraint in 01-project-structure.md:
the Boot 4.1 plugin needs 8.14+ or 9.x while KGP 2.4.10 tops out at 9.5.0, so
bumping either side means recomputing the intersection.

Skeleton follows the existing convention (## 决策 first, then 使用规则, topic
sections, ending with a 为什么不选另一个方案 comparison table).

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

README.md

@@ -106,6 +106,7 @@ V1.1 相对 V1.0 的主要变化：第 4 章拆分到 [`prd/modules/`](./prd/mod
 | [backend/10-testing.md](./backend/10-testing.md) | 测试策略（Testcontainers、WireMock、ArchUnit、覆盖率聚合） |
 | [backend/11-cross-domain-collaboration.md](./backend/11-cross-domain-collaboration.md) | 跨域协作与聚合（契约模块、领域事件、并行 fan-out 与局部降级） |
 | [backend/12-concurrency-and-scheduling.md](./backend/12-concurrency-and-scheduling.md) | 并发、事务与定时任务（事务边界、幂等、乐观锁、ShedLock、本地缓存） |
+| [backend/13-kotlin-style.md](./backend/13-kotlin-style.md) | Kotlin 语法与编码约束（只当语法糖用不当新范式用；禁 `!!`/`lateinit`/协程，Entity 禁 data class） |
 
 ### POC/