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.

docs/LAUNCHER.md

@@ -559,3 +559,13 @@ var updateCheckService = new UpdateCheckService(
 - `apply-update`, `plugin-install`, and `debug-preview` must not auto-enter OOBE.
 - Allowed elevation paths are limited to the installer itself, full installer update application, and user-confirmed legacy uninstall.
 - Default plugin installation targets the current user's LocalAppData scope and must not request elevation by default.
+
+## Public IPC Baseline
+
+Launcher now consumes Host startup telemetry from the unified public IPC stack:
+
+- Host publishes `StartupProgressMessage` via `lanmountain.launcher.startup-progress`
+- Host publishes `LoadingStateMessage` via `lanmountain.launcher.loading-state`
+- Launcher connects through `LanMountainDesktopIpcClient`
+
+The previous custom length-prefixed named-pipe transport is no longer the primary startup communication path.