Add external public IPC host/client and plugin SDK

Introduce a new LanMountainDesktop.Shared.IPC project implementing a public IPC host and client (LanMountainDesktopIpcClient, PublicIpcHostService), IPC constants and routed notify IDs, DTOs and DI helpers for registering public services. Update Plugin SDK to allow plugins to contribute public IPC services and registrations, add related descriptors/records and extension helpers. Migrate Launcher/App to use the new public IPC for startup/loading notifications and wiring (including TryConnect helper), switch LoadingStateReporter to use the external notification publisher, and add host-side public services (app info, shell control, plugin catalog). Include integration tests and spec/checklist/docs for the external IPC public API.

LanMountainDesktop.Shared.IPC/DependencyInjection/PublicIpcServiceRegistration.cs

@@ -0,0 +1,8 @@
+namespace LanMountainDesktop.Shared.IPC.DependencyInjection;
+
+public sealed record PublicIpcServiceRegistration(
+    Type ContractType,
+    Func<IServiceProvider, object> ImplementationFactory,
+    string? ObjectId,
+    string? PluginId,
+    string[] NotifyIds);

LanMountainDesktop.Shared.IPC/DependencyInjection/ServiceCollectionExtensions.cs

@@ -0,0 +1,83 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace LanMountainDesktop.Shared.IPC.DependencyInjection;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddLanMountainDesktopIpcHost(
+        this IServiceCollection services,
+        string? pipeName = null)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+
+        services.AddSingleton(provider =>
+        {
+            var host = new PublicIpcHostService(pipeName ?? IpcConstants.DefaultPipeName);
+            foreach (var registration in provider.GetServices<PublicIpcServiceRegistration>())
+            {
+                var implementation = registration.ImplementationFactory(provider);
+                host.RegisterPublicService(
+                    registration.ContractType,
+                    implementation,
+                    registration.ObjectId,
+                    registration.PluginId,
+                    registration.NotifyIds);
+            }
+
+            host.Start();
+            return host;
+        });
+
+        services.AddSingleton<IExternalIpcNotificationPublisher>(provider =>
+            provider.GetRequiredService<PublicIpcHostService>());
+
+        return services;
+    }
+
+    public static IServiceCollection AddPublicIpcService<TContract, TImplementation>(
+        this IServiceCollection services,
+        string? objectId = null,
+        string? pluginId = null,
+        params string[] notifyIds)
+        where TContract : class
+        where TImplementation : class, TContract
+    {
+        ArgumentNullException.ThrowIfNull(services);
+
+        EnsureSingletonRegistration<TContract, TImplementation>(services);
+
+        if (!services.Any(descriptor =>
+                descriptor.ServiceType == typeof(PublicIpcServiceRegistration) &&
+                descriptor.ImplementationInstance is PublicIpcServiceRegistration existing &&
+                existing.ContractType == typeof(TContract) &&
+                string.Equals(existing.ObjectId, objectId, StringComparison.Ordinal)))
+        {
+            services.AddSingleton(new PublicIpcServiceRegistration(
+                typeof(TContract),
+                provider => provider.GetRequiredService<TContract>(),
+                objectId,
+                pluginId,
+                notifyIds ?? []));
+        }
+
+        return services;
+    }
+
+    private static void EnsureSingletonRegistration<TContract, TImplementation>(IServiceCollection services)
+        where TContract : class
+        where TImplementation : class, TContract
+    {
+        var descriptor = services.LastOrDefault(item => item.ServiceType == typeof(TContract));
+        if (descriptor is null)
+        {
+            services.AddSingleton<TContract, TImplementation>();
+            return;
+        }
+
+        if (descriptor.Lifetime != ServiceLifetime.Singleton)
+        {
+            throw new InvalidOperationException(
+                $"Public IPC contract '{typeof(TContract).FullName}' must be registered as Singleton.");
+        }
+    }
+}