dvui: run-dvui on dvui.osWindow, plain dvui code

[foxnne]

Part of fizzyedit/fizzy#281. Draft: it pins fizzy at fizzyedit/fizzy#366's commit, and gets repinned to main once that merges.

run-dvui is now plain dvui code. It opens its windows with dvui's own dvui.osWindow(...): a title, a size, a min size, an open_flag. On fizzy's backend each becomes an OS window drawn from the one dvui.Window, with dvui's header and close button and no OS title bar (fizzyedit/fizzy#366). There's no viewports code in the app any more: about 270 lines of windowing became about 100 lines of ordinary dvui (−236 / +43).

dvui's demo stays a floating window in the main window, as it is in dvui's own example app. Only dvui.osWindow asks for an OS window.

-Ddvui-backend=sdl3: the same code on dvui's own SDL3 backend, where dvui.osWindow opens a dvui.Window of its own, as dvui does anywhere.

Verification, macOS, from the URL pin:

• zig build dvui on both backends, the studio shape and replay build.
• On fizzy's backend, two windows open as OS windows of their own, cascaded 24 pt apart over the main window. No warnings, and it quits cleanly on SIGTERM.

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