🧩 Refactor(Workflow): 抽取ControlFlowArms共享值对象-KISS优化阶段L5

新增 ControlFlowArms 值对象（Contract 层），集中控制流 arm 的访问逻辑：
Arms/TrueBlockName/FalseBlockName/LoopbackTarget/SetArm。消除 CFGStatement 与
FlowControlStatement 两处约 80 行逐字/近字重复，并修复一个隐藏的
null-vs-string.Empty 不一致（TrueBlockName/FalseBlockName 便利访问器在两层
返回不同类型的"空"）。

ControlFlowArms 统一空值约定为 string.Empty（经 SetArm 归一化），避免 null
泄漏进 SourceCode 拼接。

# KitX Workflow KISS 深度重构 L5 阶段：抽取共享值对象（子模块部分）

# Date: 2026-06-19
# Author: StarInk

Author: StardustSeemsInk

KitX Core Contracts/KitX.Core.Contract/Workflow/ControlFlowArms.cs

@@ -0,0 +1,73 @@
+using System.Collections.Generic;
+
+namespace KitX.Core.Contract.Workflow;
+
+/// <summary>
+/// Shared control-flow arms model embedded by both <c>CFGStatement</c> (CFG layer) and
+/// <c>FlowControlStatement</c> (BlockScript layer). Eliminates the ~80 lines of
+/// character-identical <c>Arms</c>/<c>TrueBlockName</c>/<c>FalseBlockName</c>/
+/// <c>LoopbackTarget</c>/<c>SetArm</c> duplication that had drifted between the two layers
+/// (including a hidden null-vs-string.Empty inconsistency in the convenience accessors).
+///
+/// <para>Both layers expose this via their own delegating properties (<c>Arms</c>,
+/// <c>TrueBlockName</c>, <c>FalseBlockName</c>, <c>LoopbackTarget</c>) so existing call
+/// sites (<c>stmt.Arms</c>, <c>flow.TrueBlockName</c>, ...) are unchanged.</para>
+///
+/// <list type="bullet">
+/// <item>Branch: [<c>"True"</c>, <c>"False"</c>]</item>
+/// <item>Loop: [<c>"LoopBody"</c>, <c>"LoopEnd"</c>]</item>
+/// <item>ToLoopCond: [<c>"Exec"</c> with <see cref="BranchArm.IsLoopback"/>=true]</item>
+/// <item>Switch: [<c>"Default"</c>, <c>"0"</c>, <c>"1"</c>, ... , <c>"N-1"</c>]</item>
+/// </list>
+/// </summary>
+public class ControlFlowArms
+{
+    /// <summary>
+    /// Outgoing arms of this control-flow statement. Generalised model replacing the former
+    /// fixed <c>TrueBlockName</c>/<c>FalseBlockName</c>/<c>ToLoopCondReturnTo</c> triple.
+    /// See <see cref="BranchArm"/> for the per-arm layout of each control-flow kind.
+    /// </summary>
+    public List<BranchArm> Arms { get; set; } = [];
+
+    /// <summary>Convenience accessor: the true-branch / loop-body target (Arms[0]).</summary>
+    public string? TrueBlockName
+    {
+        get => Arms.Count > 0 ? Arms[0].TargetBlockName : null;
+        set => SetArm(0, "True", value);
+    }
+
+    /// <summary>Convenience accessor: the false-branch / loop-exit target (Arms[1]).</summary>
+    public string? FalseBlockName
+    {
+        get => Arms.Count > 1 ? Arms[1].TargetBlockName : null;
+        set => SetArm(1, "False", value);
+    }
+
+    /// <summary>
+    /// The ToLoopCond loopback target — the loop condition block this statement returns to.
+    /// Unified into <see cref="Arms"/>[0] (PinName="Exec", IsLoopback=true) so ToLoopCond is
+    /// treated uniformly with Branch/Loop/Switch: all control-flow targets live in Arms.
+    /// The former standalone <c>ToLoopCondReturnTo</c> field was a patch over an Arms[0]
+    /// collision that no longer exists; Loop statements no longer carry this metadata at all
+    /// (it was dead — never read for control flow, only copied and debug-printed).
+    /// </summary>
+    public string? LoopbackTarget
+    {
+        get => Arms.Count > 0 ? Arms[0].TargetBlockName : null;
+        set => SetArm(0, "Exec", value, isLoopback: true);
+    }
+
+    /// <summary>
+    /// Sets the arm at <paramref name="index"/>, growing <see cref="Arms"/> with blank arms
+    /// as needed. Normalises a null <paramref name="value"/> to <c>string.Empty</c> so the
+    /// convenience accessors never surface null into SourceCode concatenation.
+    /// </summary>
+    public void SetArm(int index, string pinName, string? value, bool isLoopback = false)
+    {
+        while (Arms.Count <= index)
+            Arms.Add(new BranchArm());
+        Arms[index].PinName = pinName;
+        Arms[index].TargetBlockName = value ?? string.Empty;
+        Arms[index].IsLoopback = isLoopback;
+    }
+}