💥 feat(planning)!: 拆分 C# ICSP 并默认 Rust

* 新增可选 Flyshot.CSharp.Planning 项目

* 将 icsp 映射到 Rust 后端并拒绝旧 icsp_rust

* 补充后端注册、文档、调试页和回归测试

BREAKING CHANGE: method=icsp 现在默认调用 Rust 后端；C# ICSP 改为 method=icsp-csharp，旧 method=icsp_rust 不再作为正式入口。

tests/Flyshot.Core.Tests/PlanningCompatibilityTests.cs

@@ -1,4 +1,5 @@
 using Flyshot.Core.Config;
+using Flyshot.CSharp.Planning;
 using Flyshot.Core.Domain;
 using Flyshot.Core.Planning;
 using Flyshot.Core.Planning.Sampling;
@@ -27,7 +28,7 @@ public sealed class PlanningCompatibilityTests
                     new[] { 0.8, 0.3, 0.0, 0.0, 0.0, 0.0 },
                     new[] { 1.0, 0.2, 0.0, 0.0, 0.0, 0.0 }
                 }),
-            method: PlanningMethod.Icsp);
+            method: PlanningMethod.IcspCSharp);
 
         var trajectory = new ICspPlanner().Plan(request);
 
@@ -51,7 +52,7 @@ public sealed class PlanningCompatibilityTests
                     new[] { 20.0 },
                     new[] { 30.0 }
                 }),
-            method: PlanningMethod.Icsp);
+            method: PlanningMethod.IcspCSharp);
 
         var planner = new ICspPlanner(maxIterations: 0);