Harden OOBE, launch-source and elevation flow

Introduce a per-user OOBE state model and hardened launch/elevation handling. Adds OobeStateFile/OobeLaunchDecision models, OobeStateService (persisting %LOCALAPPDATA%/.launcher/state/oobe-state.json), and LauncherExecutionContext to capture elevation and user SID. CommandContext now normalizes/infers launch-source values (normal, postinstall, apply-update, plugin-install, debug-preview) and exposes maintenance checks. LauncherFlowCoordinator propagates richer launcher context details for diagnostics and suppresses OOBE for elevated/maintenance contexts. PluginInstallerService avoids requesting elevation for user-scoped installs and returns a clear error when installation target is outside the current user's LocalAppData. LauncherClient maps and surfaces result codes, UpdateWorkflow and installer invocation now pass explicit --launch-source values, and WelcomeOobeStep persists OOBE completion via the new service. Adds unit tests (CommandContext, OobeStateService, PluginInstallerService), docs/specs/checklists for the contract, and makes internals visible to tests.

LanMountainDesktop.Launcher/Services/LauncherExecutionContext.cs

@@ -0,0 +1,30 @@
+using System.Security.Principal;
+using LanMountainDesktop.Launcher.Models;
+
+namespace LanMountainDesktop.Launcher.Services;
+
+internal static class LauncherExecutionContext
+{
+    public static LauncherExecutionSnapshot Capture()
+    {
+        var userName = Environment.UserName ?? string.Empty;
+        if (!OperatingSystem.IsWindows())
+        {
+            return new LauncherExecutionSnapshot(false, userName, null);
+        }
+
+        try
+        {
+            using var identity = WindowsIdentity.GetCurrent();
+            var principal = new WindowsPrincipal(identity);
+            return new LauncherExecutionSnapshot(
+                principal.IsInRole(WindowsBuiltInRole.Administrator),
+                userName,
+                identity.User?.Value);
+        }
+        catch
+        {
+            return new LauncherExecutionSnapshot(false, userName, null);
+        }
+    }
+}