Document installer startup failure handling and recovery

content/user/installation.md

@@ -29,7 +29,8 @@ A working native graphics driver, writable project directory and space for proje
 
 ## Installation and startup problems
 
-- **Optional tool install failed:** inspect Setup/Language Toolchain Manager output and Windows Package Manager availability. Start with bundled Lua; other languages can wait.
+- **Optional tool install failed:** Setup runs one hidden PowerShell process for all selected languages and skips already-installed tools. A host initialization failure stops the batch and reports one optional-toolchain error; the engine remains installed. Retry later from **Language Toolchain Manager** after repairing the failing PowerShell or Microsoft App Installer installation. Setup does not require Windows Terminal.
+- **Error `142` or `0xc0000142` during Setup:** retain the complete message; these are distinct codes. The failure dialog shows the launch error or exit code and the Setup log path. Setup automatically creates `Setup Log ...txt` in `%TEMP%`; run the installer with `/LOG="C:\path\setup.log"` to choose a location. If PowerShell started, its transcript is in the original user's `%LOCALAPPDATA%\Rustic Game Engine\logs\toolchain-install.log`. If that file is absent, the Setup log still records PowerShell startup failures. Start with bundled Lua while diagnosing optional tools.
 - **Tool installed but not discovered:** restart Rustic after changing PATH. Use Project Manager **Settings** and the editor Console to inspect language availability.
 - **Access denied:** choose a writable project directory. Avoid a protected installation location; check the actual OS permission error.
 - **Editor does not launch or the viewport stays blank:** inspect Project Manager **Diagnostics**, then editor Console output. Check graphics-driver/backend errors and [Troubleshooting](/docs/troubleshooting).

docs/GAMEPLAY_PROGRAMMING/external-process-languages/toolchains.md

@@ -10,3 +10,19 @@ On Windows, Setup offers these external toolchains as optional Gameplay Language
 installs every selected item through Windows Package Manager. Lua, JavaScript, and
 HTML/CSS are bundled. Additional toolchains can be installed later using **Rustic Game
 Engine > Language Toolchain Manager** in the Start menu or by rerunning Setup.
+
+Setup uses one hidden PowerShell process for all selected toolchains and skips tools
+that are already installed. A PowerShell launch failure or a WinGet initialization
+failure stops the batch instead of retrying the failing host for every language.
+Rustic itself remains installed; you can start with the bundled languages and retry
+optional tools later from **Language Toolchain Manager**.
+
+If Setup reports error `142` or `0xc0000142`, keep the full message: these are distinct
+codes, and the failing executable matters. The Setup failure dialog includes the
+launch error or exit code and the path to its log. Setup saves a log in `%TEMP%`
+(`Setup Log ...txt`) automatically; use `/LOG="C:\path\setup.log"` to choose a location.
+If PowerShell started, check the original user's
+`%LOCALAPPDATA%\Rustic Game Engine\logs\toolchain-install.log` for the failed package
+and WinGet output. If no transcript exists, inspect the Setup log for a PowerShell
+startup failure. Repair the failing PowerShell or Microsoft App Installer installation
+before retrying; opening Windows Terminal is not required by Setup.