Support multiple windows

[botoddly]

Problem

SDL3 supports multiple windows via SDL_ClaimWindowForGPUDevice, but GameKit has several single-window assumptions baked in.

1. Window is a hard singleton

GameKitAppBuilder registers one Window and aliases it as IWindow. There is no window collection or registry.

2. EventService ignores windowID when routing window events

SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED is forwarded unconditionally to the single _window without checking evt.window.windowID. With multiple windows, any resize would notify the wrong instance.

3. DefaultRenderContextProvider acquires a swapchain from one window per frame

Each window needs its own command buffer and swapchain texture acquired and submitted independently within the same frame.

4. GraphicsPipelineBuilder.AddColorFormatFromDisplay() queries the single window

It reads _window.ColorTargetFormat implicitly. Pipelines targeting a secondary window would use the wrong format.

5. ITextInputService.Start() / Stop() target a hardcoded window

TextInputService holds a fixed Window and passes it to SDL_StartTextInput / SDL_StopTextInput. PencuilRenderPhase already receives the correct IWindow for the window it renders to but has no way to pass it through.

Possible approach

• Introduce IWindowManager with a PrimaryWindow, a window collection, and CreateWindow / DestroyWindow. The existing IWindow singleton remains valid for single-window apps.
• Route SDL_EVENT_WINDOW_PIXEL_SIZE_CHANGED (and other window-specific events) through IWindowManager by windowID.
• Extend render orchestration to iterate all windows and acquire/submit a swapchain per window per frame.
• Add IWindow parameters to ITextInputService.Start(IWindow) / Stop(IWindow) so Pencuil can pass the window it rendered to.
• Change AddColorFormatFromDisplay() to accept an explicit IWindow.