Introduce install checkpoint support and resume logic for updates, plus related locking and validation. Adds InstallCheckpoint model, AppJsonContext serialization, and UpdatePaths helpers for deployment lock, apply-in-progress lock and install-checkpoint path. UpdateEngineService gains checkpoint load/save/delete, incoming-state validation, resume logic for PLONDS and legacy updates, apply lock handling, and safer cleanup; ApplyPendingPlondsUpdateAsync and ApplyPendingUpdate flow updated accordingly. Add DeploymentLock contract and extend UpdateState with pause/resume/cancel helpers. Tests updated to cover stale/valid checkpoint resume and legacy/PLONDS flows. CI: enhance ddss-publish to detect release channel, validate S3 assets, prepare and atomically publish channel pointer; add ddss-rollback workflow to publish rollbacks; adjust plonds-build concurrency and release events.
Introduce a new update subsystem: shared contracts for manifests, messages, paths and state (LanMountainDesktop.Shared.Contracts.Update). Add IPC and reporting infrastructure for installer progress (IUpdateProgressReporter, LauncherUpdateProgressIpcServer, NullUpdateProgressReporter) and integrate progress/complete reporting into UpdateEngineService. Add multiple update service components and providers (CompositeManifestProvider, GithubReleaseManifestProvider, PlondsApiManifestProvider, UpdateDownloadEngine, UpdateOrchestrator, UpdateInstallGateway, CLI launcher bridge, launcher bridge interfaces, observable helper, state store, progress subject, JSON context). Update settings and models to support UseGhProxyMirror and PLONDS/GitHub fallback logic, plus localization strings and UI/viewmodel files for update settings and progress. Misc: installer script tweak and a small change in Plonds generator. This adds end-to-end support for checking, downloading and reporting update progress and results.
