chore: initial project import

backend/fleet/src/Fleet.Core/StringId.cs

@@ -0,0 +1,38 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Fleet.Core;
+
+/// <summary>节点 ID（带类型前缀的 ULID）。Monitor 中的 AgentId 就是 NodeId。</summary>
+[JsonConverter(typeof(NodeIdJsonConverter))]
+public readonly record struct NodeId(string Value)
+{
+    public override string ToString() => Value;
+}
+
+/// <summary>强类型字符串 ID 的 JSON 转换器基类（源生成友好，不使用反射）。</summary>
+public abstract class StringIdJsonConverter<TId> : JsonConverter<TId>
+{
+    protected abstract TId Create(string value);
+
+    protected abstract string GetValue(TId id);
+
+    public sealed override TId Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
+        Create(reader.GetString() ?? throw new JsonException("id must be a string"));
+
+    public sealed override void Write(Utf8JsonWriter writer, TId value, JsonSerializerOptions options) =>
+        writer.WriteStringValue(GetValue(value));
+
+    public sealed override TId ReadAsPropertyName(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
+        Create(reader.GetString() ?? throw new JsonException("id must be a string"));
+
+    public sealed override void WriteAsPropertyName(Utf8JsonWriter writer, TId value, JsonSerializerOptions options) =>
+        writer.WritePropertyName(GetValue(value));
+}
+
+public sealed class NodeIdJsonConverter : StringIdJsonConverter<NodeId>
+{
+    protected override NodeId Create(string value) => new(value);
+
+    protected override string GetValue(NodeId id) => id.Value;
+}