windows: menus and dialogs in windows of their own, as Acrylic popups

[foxnne]

Part of #226 (docs/NATIVE_WINDOWS_PLAN.md); step 3 of docs/WINDOWS_LINUX_GLASS_PLAN.md (#236).

What changes

With floats as windows (FIZZY_POPOUT=1 on Windows), menus and dialogs now leave the main window as they do on macOS (viewports.menus). Each one is a borderless window owned by the window it opens from, never activated, and dressed by DWM (win32_titlebar.viewportMenuChrome):

• Acrylic (DWMSBT_TRANSIENTWINDOW, the material of Windows 11's own menus);
• DWM's rounded corners and shadow;
• no border, no system menu, no DWM transitions.

DWM draws a never-activated window's backdrop as its solid fallback, so the menu's window is kept looking active (WM_NCACTIVATE). An owned window isn't carried when its owner moves, so a menu's window rides on nothing and is placed each frame.

SDK impact

• None

Verified

Windows 11 ARM VM (fizzy-win11, WARP, aarch64-windows-gnu Debug), driven with real input (SendInput) and checked against the window list (EnumWindows plus DWM attributes) and screenshots:

• The File and Help menus and the "Unsaved changes" dialog each open in a window of their own:
  • owned by the main window and directly above it in the z-order;
  • WS_EX_NOACTIVATE, no WS_SYSMENU;
  • backdrop type Acrylic (3), corner preference round;
  • the main window stays the foreground window.
• The first click on an item acts: New File opens untitled-1, and Open Files opens the Open dialog. A temporary log showed the press arriving on the menu's window and mapping into the menu's dvui subwindow, not what lies under it.
• The dialog's window hides when the main window is minimized and comes back on restore.
• Gates: zig build, test, test-integration, check-web, test-sdk-version, and the Windows and Linux cross-builds.
• The look on a hardware GPU. The VM's DWM draws every backdrop as its fallback, so these need a real machine:
  • Acrylic rather than a solid colour;
  • rounded corners and a shadow;
  • whether the WM_NCACTIVATE trick keeps the Acrylic.
• A menu opened over a float's window. Not exercised; creating a float needs a view drag.
• macOS / Linux / Web: unchanged (viewports.menus was already on for macOS, and this is the Windows branch).

Seen while testing, not caused by this change:

• A menu closes when fizzy is deactivated. My harness's own process start deactivated it, which made a later click fall through to the main window.
• A dialog drifts a few pixels left over each minimize/restore. It does the same drawn in-window (FIZZY_NATIVE_DIALOGS=0).

Follow-ups

• NATIVE_WINDOWS_PLAN's Phase 1 on Windows: the slider along DWM's backdrop types, one activation group, and a policy probe.
• Caption buttons in a Windows float's header (in app/layout/Floats.zig).

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