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.

.trae/specs/external-ipc-public-api/checklist.md

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+# External IPC Public API Checklist
+
+- [x] Host can expose strong-typed public IPC services.
+- [x] External .NET client can connect and call built-in services.
+- [x] Host publishes launcher startup and loading-state notifications through routed notify.
+- [x] Launcher consumes routed notify instead of the old primary custom named-pipe path.
+- [x] Plugin SDK exposes public IPC contribution primitives.
+- [x] Plugin runtime can discover and register plugin public IPC services.
+- [x] Public catalog includes built-in and plugin-contributed services.
+- [x] `catalog.changed` is emitted when new services are added after startup.
+- [ ] Add example external client sample.

.trae/specs/external-ipc-public-api/spec.md

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+# External IPC Public API Spec
+
+## Goal
+
+Provide a single `dotnetCampus.Ipc` based external integration layer for:
+
+- Host public APIs
+- Launcher/OOBE startup progress and loading-state notifications
+- plugin-contributed public services and live event push
+
+## Delivered
+
+- `LanMountainDesktop.Shared.IPC` project
+- `[IpcPublic]` based built-in public contracts
+- `PublicIpcHostService` and `LanMountainDesktopIpcClient`
+- Launcher migrated to Host public IPC notifications
+- Plugin SDK public IPC contribution API
+- Host runtime integration for plugin public IPC services
+
+## Out of Scope
+
+- plugin process isolation
+- non-.NET strong-typed public IPC clients
+- live plugin public service removal without restart

.trae/specs/external-ipc-public-api/tasks.md

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+# External IPC Public API Tasks
+
+- [x] Add `LanMountainDesktop.Shared.IPC`
+- [x] Expose built-in `[IpcPublic]` services
+- [x] Add routed notify constants and public IPC client/host wrappers
+- [x] Start Host public IPC during app startup
+- [x] Move Launcher startup progress consumption to the new IPC base
+- [x] Add plugin public IPC registration/contributor SDK
+- [x] Register plugin-contributed public services into Host catalog
+- [x] Add integration tests for strong-typed public service access and plugin registration descriptors
+- [ ] Expand built-in public service surface beyond the first minimal set
+- [ ] Add non-.NET bridge guidance and samples