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.

.trae/specs/launcher-upgrade/spec.md

@@ -52,3 +52,9 @@ Upgrade `LanMountainDesktop.Launcher` into the unified Launcher for:
 
 - `IOobeStep` for future multi-step OOBE
 - `ISplashStageReporter` for future startup progress visualization
+
+## Compatibility Addendum
+
+- The current production OOBE state format is a per-user JSON file at `%LOCALAPPDATA%\LanMountainDesktop\.launcher\state\oobe-state.json`.
+- `first_run_completed` remains legacy compatibility data only.
+- Same-user reinstall or upgrade should not re-enter OOBE.