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.Tests/CommandContextTests.cs

@@ -0,0 +1,25 @@
+using LanMountainDesktop.Launcher;
+using Xunit;
+
+namespace LanMountainDesktop.Tests;
+
+public sealed class CommandContextTests
+{
+    public static TheoryData<string[], string> LaunchSourceCases => new()
+    {
+        { [], "normal" },
+        { ["preview-oobe"], "debug-preview" },
+        { ["apply-update"], "apply-update" },
+        { ["--source", "plugin.lmdp", "--plugins-dir", "plugins", "--result", "result.json"], "plugin-install" },
+        { ["launch", "--launch-source", "postinstall"], "postinstall" }
+    };
+
+    [Theory]
+    [MemberData(nameof(LaunchSourceCases))]
+    public void FromArgs_InfersExpectedLaunchSource(string[] args, string expectedLaunchSource)
+    {
+        var context = CommandContext.FromArgs(args);
+
+        Assert.Equal(expectedLaunchSource, context.LaunchSource);
+    }
+}