Add basic ID classes and parsing

.gitmodules

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+[submodule ".gts-spec"]
+	path = .gts-spec
+	url = https://github.com/GlobalTypeSystem/gts-spec.git

Gts.Application/Gts.Application.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <RootNamespace>Gts.Application</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <FrameworkReference Include="Microsoft.AspNetCore.App" />
+    <ProjectReference Include="..\Gts.Store\Gts.Store.csproj" />
+  </ItemGroup>
+
+</Project>

Gts.Application/GtsEntityOperations.cs

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+using System.Text.Json.Nodes;
+using Gts.Extraction;
+using Gts.Store;
+
+namespace Gts.Application;
+
+/// <summary>Adds entities to a registry with the same rules as the GTS HTTP API.</summary>
+public static class GtsEntityOperations
+{
+    public sealed record AddResult(bool Ok, string Id, string? SchemaId, bool IsSchema, string? Error);
+
+    public static async Task<AddResult> TryAddAsync(
+        GtsRegistry registry,
+        JsonObject body,
+        bool validate,
+        GtsExtractOptions? extractOptions = null,
+        CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        var opt = extractOptions ?? GtsExtractOptions.Default;
+        var entity = GtsJsonEntity.ExtractEntity(body, opt);
+        var extract = GtsJsonEntity.ExtractId(body, opt);
+
+        if (!entity.IsSchema)
+        {
+            if (string.IsNullOrEmpty(entity.SelectedEntityField))
+                return new AddResult(false, "", null, false, "Instance must have an id field");
+        }
+        else
+        {
+            if (entity.GtsId is null)
+                return new AddResult(false, "", null, true, "Unable to detect GTS ID in schema");
+        }
+
+        if (validate && entity.IsSchema)
+        {
+            var rawId = body.TryGetPropertyValue("$id", out var idn) ? idn?.GetValue<string>() : null;
+            if (!string.IsNullOrEmpty(rawId) && rawId.StartsWith("gts.", StringComparison.Ordinal) &&
+                !rawId.StartsWith("gts://", StringComparison.Ordinal))
+                return new AddResult(false, "", null, true, "Schema $id must use gts:// URI format, not plain gts. prefix");
+        }
+
+        try
+        {
+            GtsSchemaRefFormatValidator.ValidateRefs(body);
+        }
+        catch (Exception ex)
+        {
+            return new AddResult(false, "", null, entity.IsSchema, ex.Message);
+        }
+
+        if (entity.IsSchema && entity.GtsId is not null)
+        {
+            var schemaVr = await registry.ValidateSchemaAsync(entity.GtsId, entity.Content, cancellationToken)
+                .ConfigureAwait(false);
+            if (!schemaVr.Ok)
+            {
+                var msg = schemaVr.Errors is { Count: > 0 }
+                    ? string.Join("; ", schemaVr.Errors)
+                    : (schemaVr.FailureReason ?? "Schema validation failed");
+                return new AddResult(false, entity.GtsId.Id, entity.SchemaId, true, msg);
+            }
+
+            await registry.SaveAsync(entity).ConfigureAwait(false);
+
+            var idOut = entity.GtsId.Id;
+            return new AddResult(true, idOut, string.IsNullOrEmpty(entity.SchemaId) ? null : entity.SchemaId, true, null);
+        }
+
+        await registry.SaveAsync(entity).ConfigureAwait(false);
+
+        if (validate && !entity.IsSchema && entity.GtsId is not null)
+        {
+            var vr = await registry.ValidateInstanceAsync(entity.GtsId.Id, cancellationToken).ConfigureAwait(false);
+            if (!vr.Ok)
+                return new AddResult(false, entity.GtsId.Id, entity.SchemaId, false, vr.FailureReason ?? "Validation failed");
+        }
+
+        var idOut = entity.GtsId?.Id ?? extract.Id ?? "";
+        return new AddResult(true, idOut, string.IsNullOrEmpty(entity.SchemaId) ? null : entity.SchemaId, entity.IsSchema, null);
+    }
+}