Windowing foundation: one model, decided once a frame, tested without a screen #243

The plan is docs/WINDOWING_FOUNDATION_PLAN.md (#246). Every step keeps today's behaviour. They make the windowing (floats, menus, dialogs, view drags, drop zones) testable without a screen and cheap in native calls, share one model across platforms, and leave the backend usable by any dvui app. It builds on NATIVE_WINDOWS_PLAN.md and WINDOWS_LINUX_GLASS_PLAN.md, and doesn't repeat them.

Findings from the 2026-10-08 review are in the doc. In short:

• one Viewport struct serves five kinds of window;
• Popout re-decides every window by hand each frame, through five near-copies of fill-and-present;
• there are seven coordinate spaces and two different band thresholds;
• drag hit tests repeat once per region per frame;
• macOS makes about 20 Objective-C messages per float per frame, changed or not;
• glass looks are mirrored four deep and the shader is hand-synced three ways;
• 32 layout tests never ran;
• nothing popped out is tested.

Steps, each its own PR:

• 1. Run the app module's own tests (32 that never ran): layout: run the app module's own tests #244
• 2. Menus and dialogs stay auxiliary windows on macOS: macos: menus and dialogs stay auxiliary windows #245
• 3. Typed coordinate spaces and one bandOf in viewport_map, after linux: the desktop's blur behind the window, behind FIZZY_BLUR_BEHIND=1 #237 and windows: menus and dialogs in windows of their own, as Acrylic popups #238
• 4. OsWindow: kinds as a union, native handles cached, dressing applied only on change, per-kind budgets (measure Objective-C messages per frame before and after)
• 5. The frame's windows as a pure plan the backend reconciles, plus a fake backend and the first integration tests that pop a float out
• 6. One fill for every OS window, with a target pool
• 7. A drag model computed once a frame, with tests, and the release rules in one table
• 8. Float, window and drag lifecycles as explicit states
• 9. Glass: published capabilities, one look record end to end, shader parity (after core: Vulkan and D3D12 glass draws the lens and follows the slider #242)
• 10. Full-screen transitions driven by Core Animation, if release builds don't hold 8.3 ms
• 11. dvui changes on the fork (subwindow render redirection, a screens hook, subwindow kind and parent), the backend as a package, and dvui's example app as the acceptance test
• 12. Remove the dead viewport API, along with steps 3–5