🧩 Refactor(Contract): 新增 BranchArm 与 VariadicPinSpec,扩展 NodeDescriptor

- 新增 BranchArm:通用控制流臂模型(PinName/TargetBlockName/IsLoopback),
  位于 Contract 层供 Workflow 与 Contract 共享
- 新增 VariadicPinSpec record:声明节点的变长端口增长规则
  (BasePinName/StartIndex/PinType),供编辑器泛化"连一个长一个"机制
- NodeDescriptor 新增可选 InputVariadic/OutputVariadic 字段(默认 null=固定)

Author: StardustSeemsInk

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

@@ -0,0 +1,28 @@
+namespace KitX.Core.Contract.Workflow;
+
+/// <summary>
+/// A single outgoing arm of a control-flow statement (Branch / Loop / ToLoopCond / Switch).
+/// Replaces the former fixed <c>TrueBlockName</c>/<c>FalseBlockName</c>/<c>ToLoopCondReturnTo</c>
+/// triple, generalising the model to any number of arms so that an N-way <c>Switch</c>
+/// (and future control-flow builtins) can be expressed without positional hacks.
+/// </summary>
+public class BranchArm
+{
+    /// <summary>
+    /// The output pin name this arm corresponds to on the Blueprint node
+    /// (e.g. <c>"True"</c>, <c>"False"</c>, <c>"LoopBody"</c>, <c>"LoopEnd"</c>,
+    /// <c>"Exec"</c>, <c>"Default"</c>, or <c>"0"</c>..<c>"N-1"</c> for Switch).
+    /// </summary>
+    public string PinName { get; set; } = string.Empty;
+
+    /// <summary>
+    /// The target block name this arm transfers control to.
+    /// </summary>
+    public string TargetBlockName { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Whether this arm is a loopback edge (used by <c>ToLoopCond</c>, which returns
+    /// to the parent loop's condition block). Drives <c>CFGEdgeType.LoopbackToCondition</c>.
+    /// </summary>
+    public bool IsLoopback { get; set; }
+}

KitX Core Contracts/KitX.Core.Contract/Workflow/Nodes/NodeDescriptor.cs

@@ -6,10 +6,14 @@ namespace KitX.Core.Contract.Workflow;
 /// Describes a node type's layout and pin configuration.
 /// Each node subclass provides its own descriptor via GetDescriptor().
 /// </summary>
+/// <param name="InputVariadic">Optional variadic-growth spec for input pins. Null = fixed.</param>
+/// <param name="OutputVariadic">Optional variadic-growth spec for output pins. Null = fixed.</param>
 public record NodeDescriptor(
     double Width,
     double Height,
     IReadOnlyList<PinDescriptor> InputPins,
     IReadOnlyList<PinDescriptor> OutputPins,
-    string DisplayName
-);
+    string DisplayName,
+    VariadicPinSpec? InputVariadic = null,
+    VariadicPinSpec? OutputVariadic = null
+);

KitX Core Contracts/KitX.Core.Contract/Workflow/Nodes/VariadicPinSpec.cs

@@ -0,0 +1,22 @@
+namespace KitX.Core.Contract.Workflow;
+
+/// <summary>
+/// Declares a variadic (auto-growing) pin group on a node descriptor.
+/// <para>
+/// When a node declares <c>InputVariadic</c> or <c>OutputVariadic</c>, the Blueprint editor
+/// appends a fresh pin of <see cref="PinType"/> whenever the <em>last</em> pin of that group
+/// gets connected — so the user can chain more inputs/outputs without manually adding pins.
+/// </para>
+/// <para>
+/// <see cref="BasePinName"/> is the title prefix for new pins; <see cref="StartIndex"/> is the
+/// starting number appended to it. The editor derives each new pin's name from the node's
+/// current pin count (not by mutating this record), so every node instance counts independently
+/// and survives save/load round-trips.
+/// </para>
+/// <para>Examples:</para>
+/// <list type="bullet">
+/// <item>StringConcat input: <c>new("Input ", 3, PinType.String)</c> → "Input 3", "Input 4", ...</item>
+/// <item>Switch output: <c>new("", 1, PinType.Execution)</c> → "1", "2", ...</item>
+/// </list>
+/// </summary>
+public record VariadicPinSpec(string BasePinName, int StartIndex, PinType PinType);