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>

backend/01-project-structure.md

@@ -12,6 +12,7 @@ Kotlin + Spring Boot + Gradle（Groovy DSL，`build.gradle`）。
 | Spring Cloud | `2025.1.2`（Oakwood） | Boot 4.1 需要 2025.1.2 及以上；提供 `spring-cloud-kubernetes` 5.0.2，见 [07-config-governance.md](./07-config-governance.md) |
 | Kotlin | `2.4.10` | |
 | Java | `21`（LTS，用 Gradle toolchain 锁定） | |
+| Gradle | `9.5.0` | **双向约束**：Boot 4.1 的 Gradle 插件要求 8.14+ 或 9.x，而 KGP 2.4.10 的完全支持上限是 9.5.0。升 Boot 或升 Kotlin 都要重新取一次交集——只看一边会踩到另一边的上限 |
 
 Spring Boot 4 连带升级了 Spring Framework 7 / Spring Security 7 / **Jackson 3**，第三方库必须选对应版本线，各篇文档里出现的坐标已按此对齐：