💥 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/RustIcspPlannerInteropTests.cs

@@ -1,4 +1,5 @@
 using Flyshot.Core.Domain;
+using Flyshot.CSharp.Planning;
 using Flyshot.Core.Planning;
 using Flyshot.Native.Planning;
 
@@ -35,7 +36,7 @@ public sealed class RustIcspPlannerInteropTests
 
         var result = planner.Plan(CreateSimpleRequest());
 
-        Assert.Equal(PlanningMethod.IcspRust, result.Method);
+        Assert.Equal(PlanningMethod.Icsp, result.Method);
         Assert.Equal(result.PlannedWaypointCount, result.WaypointTimes.Count);
         Assert.Equal(result.PlannedWaypointCount - 1, result.SegmentDurations.Count);
         Assert.Equal(result.SegmentDurations.Count, result.SegmentScales.Count);