feat(windows): scaffold WinUI 3 app shell consuming libghostty

[deblasis]

First pass at a native Windows shell for Ghostty, mirroring how macos/ consumes libghostty. Adds windows/Ghostty/ as a WinUI 3 unpackaged desktop app that hosts a single libghostty surface via the DX12 composition path (SwapChainPanel, null HWND).

What's in place

• Ghostty.sln + Ghostty.csproj — net9.0-windows, unpackaged. Copies ghostty.dll from zig-out/lib into a native/ subdirectory to avoid collision with the managed Ghostty.dll on case-insensitive filesystems. NativeLibrary.SetDllImportResolver loads it at startup.
• Directory.Build.props — redirects AppxMSBuildToolsPath to VS Build Tools so dotnet build from the CLI can find the PRI task DLL (WindowsAppSDK#3997).
• global.json — pins .NET 9 SDK. .NET 10 is installed and breaks the WinUI 3 build targets.
• app.manifest — PerMonitorV2 DPI, longPathAware, UTF-8 code page.
• Interop/NativeMethods.cs — P/Invoke covering init, config lifecycle, app lifecycle, surface lifecycle, size/scale/focus, key/text/mouse. Incremental subset of ghostty.h sorted in header order for easy drift detection on rebases. Runtime config callbacks held in fields so the GC does not free them while libghostty holds the function pointers.
• Interop/ISwapChainPanelNative.cs — COM interop. QueryInterface on the WinUI 3 SwapChainPanel returns the ISwapChainPanelNative v-table pointer, which is what libghostty's DX12 renderer calls SetSwapChain on (slot 3). Passing the raw IUnknown crashes inside the renderer.
• App.xaml, MainWindow.xaml — Mica backdrop, default title bar (custom title bar is a follow-up PR).
• Controls/TerminalControl.xaml(.cs) — SwapChainPanel + input routing. Fills working_directory/command/initial_input with an empty C string (not null) because Zig crashes in surface_new otherwise. Resize is debounced on a 30ms DispatcherQueueTimer because the DX12 renderer recreates the swap chain on every set_size, which can't keep up with WinUI 3's per-pixel drag events. CharacterReceived filters C0 control chars so Ctrl+key combinations don't double up (same fix as the Win32 example in 8dde86d).

just recipes

• `just build-win` — dotnet build the shell alone.
• `just run-win` — zig build the DLL, dotnet build the shell, launch it.

First-boot status

Opens a Mica window, creates the surface, spawns cmd.exe, DX12 renders, title updates, resize no longer crashes.

Known issues to address in follow-up stacked PRs:

• Focus flaps between the UserControl and the inner SwapChainPanel
• Terminal does not pixel-perfect fill the window (swap chain is sized correctly, visual clipping to investigate)
• VirtualKey is passed as the raw Keycode instead of a translated GhosttyKey

Not included yet (intentional)

Tabs, splits, custom title bar, settings UI, clipboard, bell, shell integration, dispatcher for ghostty_action callbacks, full VirtualKey -> GhosttyKey mapping, and about 70 of the 88 functions in ghostty.h. Each unlocks its own small PR.