UI.4 Studio: torn-off windows are real windows, placed on the right monitor and remembered, on Windows, macOS and Linux

In CadCraft Studio a window.open becomes a real Tauri window through on_new_window in desktop-shell src-tauri/src/lib.rs (built on about:blank; the engine loads the asked page into the window it is handed). This path has crashed Studio on Windows before (TakeoffCraft pop-up, 0.6 report) and the 0.8.1 hand-off was never tested on a Windows build. This row: the window service (UI.2/UI.3) in Studio = one Tauri window per torn-off piece with the app's icon and a title, placed on the monitor the person dragged to (Tauri monitor list, no Window Management permission prompt), its place remembered with the window ids that survive a restart (MCP-WINDOW-IDS), restored at start without a click (the desktop may do what the browser may not), closed with the main window, and a startup log line per window (PERF.18's log). Test on ws11a (two monitors) and the owner's Mac; tablets stay without pop-ups (the shell's mobile rule).