build: a dvui app of its own on fizzy's backend (fizzy.addDvui)

[foxnne]

Part of #281: what example-app's run-dvui builds on. Stacked on #326 (on #324); review those first. The example-app side is fizzyedit/example-app#4.

fizzy.addDvui(fizzy_dep, root_module, backend) gives an outside app's module dvui on one of two backends:

• .fizzy: fizzy's own, where each floating window can be an OS window of its own;
• .sdl3: dvui's own SDL3 backend, which has the main window only.

The module gets dvui (fizzy's pin), its backend, and viewports_none (#324), so the same app code builds on either:

const backend = @import("backend");
const viewports = if (@hasDecl(backend, "viewports")) backend.viewports else @import("viewports_none");

On fizzy's backend it also gets platform, and on macOS the backend's Objective-C compiled into it. build/exe.zig's backend switch is now nativeDvui, shared by fizzy's own executable and addDvui.

Two fixes a plain dvui app needed

1. The main window's drawable was acquired with a copy pass still open (GpuRenderer.acquireSwapchain). The frame's texture uploads had left it open, and SDL asserted on every frame ("Cannot acquire a swapchain texture during a pass"). Fizzy never hit this because it draws its frame into a target first. Any dvui app draws straight into the window. The fix ends the copy pass before acquiring, as presentInto and clearPass already do.
2. platform's menu-bar exports weren't emitted (FizzyMenu*). macos/menu_target.m calls them and is compiled into every macOS app on the backend, but Zig emits an export fn only when analysis reaches its file. Fizzy reaches platform/menu.zig through its menu bar; a dvui app with none failed to link. Now platform/root.zig references them, and they're pub.

Verification

• Fizzy: zig build, test (514/515, 1 skipped), test-integration (358/358), check-web, test-sdk-version, the backend's tests (46/46), and the Windows and Linux cross-builds pass on macOS. The soak tape ends in a passing verdict (1,429 viewport presents, no leaks, nothing logged as an error).
• example-app's run-dvui, built against this branch, on macOS:
  • On fizzy's backend: it opens its main window and two floating windows. Each is an OS window of its own, opened over its place in the main window (window list: main at 540,332; floats at 652,476 and 684,500, each 280×212). No assertions or warnings, and it quits cleanly on SIGTERM.
  • With -Ddvui-backend=sdl3: the same app shows only its main window, with the floating windows in it. No warnings.
  • Not seen: I couldn't screenshot, since this session has no screen-recording permission. Window positions and the logs are what I checked, not how they look.

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