💾 Feat(Blueprint): 添加 VariableNode 类型与常量覆盖执行接口

- 新增 Variable 枚举值与 VariableNode 类（无端口浮游节点，用于无初始值变量声明）
- 注册 JsonDerivedType 支持多态序列化
- IWorkflowService 新增带 constantOverrides 参数的 ExecuteBlockScriptAsync 重载

Author: StardustSeemsInk

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

@@ -82,7 +82,13 @@ public enum BlueprintNodeType
     /// <summary>
     /// Pause execution node
     /// </summary>
-    Pause
+    Pause,
+
+    /// <summary>
+    /// Variable declaration node (ConstBlock variables without initial values).
+    /// A floating node with no ports — users can only change the data type.
+    /// </summary>
+    Variable
 }
 
 /// <summary>
@@ -200,6 +206,7 @@ public class BlueprintPin
 [JsonDerivedType(typeof(SetNode), "Set")]
 [JsonDerivedType(typeof(PrintNode), "Print")]
 [JsonDerivedType(typeof(PauseNode), "Pause")]
+[JsonDerivedType(typeof(VariableNode), "Variable")]
 public abstract class BlueprintNode
 {
     /// <summary>
@@ -614,6 +621,40 @@ public PauseNode()
     );
 }
 
+/// <summary>
+/// Variable node - ConstBlock variable without initial value.
+/// A floating node with no input/output ports. Users can change the data type
+/// but not the initial value. Type changes propagate to Get/Set nodes.
+/// </summary>
+public class VariableNode : BlueprintNode
+{
+    /// <summary>
+    /// Variable name
+    /// </summary>
+    public string VarName { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Variable type (e.g., "int", "double", "string", "bool")
+    /// </summary>
+    public string VarType { get; set; } = "int";
+
+    public VariableNode()
+    {
+        NodeType = BlueprintNodeType.Variable;
+        Name = "Variable";
+        InitializePinsFromDescriptor();
+    }
+
+    public override NodeDescriptor GetDescriptor() => new(
+        Width: 120, Height: 50,
+        InputPins: [],
+        OutputPins: [],
+        DisplayName: "Variable"
+    );
+
+    public override string GetDisplayTitle() => $"Var: {VarName}";
+}
+
 /// <summary>
 /// Connection between two pins
 /// </summary>

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

@@ -147,6 +147,16 @@ Task<BlockScriptExecutionResult> ExecuteBlockScriptAsync(
         string sourceCode,
         List<HelperFunction> helperFunctions,
         System.Threading.CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Executes a block script from source code with helper functions and constant overrides.
+    /// Constant overrides replace the DefaultValue on ConstBlock variables before execution.
+    /// </summary>
+    Task<BlockScriptExecutionResult> ExecuteBlockScriptAsync(
+        string sourceCode,
+        List<HelperFunction> helperFunctions,
+        Dictionary<string, object?>? constantOverrides,
+        System.Threading.CancellationToken cancellationToken = default);
 }
 
 /// <summary>