Add install checkpoint/resume and DDSS workflows

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.

LanMountainDesktop.Launcher/Models/UpdateModels.cs

@@ -41,6 +41,25 @@ internal sealed class SnapshotMetadata
     public string Status { get; set; } = "pending";
 }
 
+internal sealed class InstallCheckpoint
+{
+    public string SnapshotId { get; set; } = string.Empty;
+
+    public string SourceVersion { get; set; } = string.Empty;
+
+    public string? TargetVersion { get; set; }
+
+    public string? SourceDirectory { get; set; }
+
+    public string TargetDirectory { get; set; } = string.Empty;
+
+    public bool IsInitialDeployment { get; set; }
+
+    public int AppliedCount { get; set; }
+
+    public int VerifiedCount { get; set; }
+}
+
 internal sealed class UpdateApplyResult
 {
     public bool Success { get; init; }