dvui: windows in the app or their own, and what they are made of

[foxnne]

Replaces #7, which GitHub closed when its base, zig-0.17, merged into main (#8) and was deleted. Same branch, with main merged in.

What the dvui example does now, on fizzy's backend (zig build dvui, plain dvui code with no fizzy app around it):

• Windows in the app, or OS windows.
  • "New window" opens an in-app floating window. Dragged by its header past the app's windows, it tears off into an OS window of its own, and dragged back over the app it goes back in (backend: a float dragged out is its own OS window from the start fizzy#577, #591, #594, #605).
  • "New OS window" opens a dvui.osWindow, an OS window from the start, with the OS's title bar in the theme's colour.
  • dvui's demo stays in the main window, as in dvui's own example, and is drawn first so it never hides the app's windows.
• What the windows are made of (when the backend has a window material, backend: plain dvui app windows made of the window material fizzy#565, #589):
  • a dropdown: Opaque, Clear, Blur, Frost, Liquid Glass;
  • a Transparency slider, which stops short of clear for glass, at the backend's floor (material_min_opacity); only Clear goes all the way.
  • The colour is the theme's window colour, from dvui's theme picker. The main window draws no fill of its own, so the material shows through.
• CI builds the dvui app beside the shapes and the replay app.

On dvui's own SDL3 backend (-Ddvui-backend=sdl3), dvui.osWindow opens a dvui.Window of its own and there's no material picker (@hasDecl guards it).

Pins: unchanged from main, fizzy's Zig 0.17 switch and SDK 0.2.30. The material picker appears once the fizzy pin includes the window material; fizzy's own CI (check-examples.sh) builds this against its current tree.

Tested:

• zig build dvui against the pins: 41/41 steps.
• By hand (the user, macOS 26) against fizzy's current tree: tear-off and back, every material, the slider, light and dark themes.

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