Phase 0 WP-0.10–0.12: Host skeleton, options classes, sample configs, and execution framework

- WP-0.10: Role-based Host startup (Central=WebApplication, Site=generic Host),
  15 component AddXxx() extension methods, MapCentralUI/MapInboundAPI stubs
- WP-0.11: 12 per-component options classes with config binding
- WP-0.12: Sample appsettings for central and site topologies
- Add execution procedure and checklist template to generate_plans.md
- Add phase-0-checklist.md for execution tracking
- Resolve all 21 open questions from plan generation
- Update IDataConnection with batch ops and IAsyncDisposable
57 tests pass, zero warnings.

src/ScadaLink.ExternalSystemGateway/ServiceCollectionExtensions.cs

@@ -0,0 +1,18 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ScadaLink.ExternalSystemGateway;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddExternalSystemGateway(this IServiceCollection services)
+    {
+        // Phase 0: skeleton only
+        return services;
+    }
+
+    public static IServiceCollection AddExternalSystemGatewayActors(this IServiceCollection services)
+    {
+        // Phase 0: placeholder for Akka actor registration
+        return services;
+    }
+}