backend: fizzy's native backend is a package of its own

[foxnne]

Part of #281

What changes

Fizzy's native backend is now a package of its own, backend/, beside sdk/. It is the first step toward a plain dvui app on fizzy's backend without the rest of fizzy (fizzyedit/example-app's run-dvui). Nothing changes in behaviour.

• Moved, unchanged: src/backend/native/** → backend/src/**, i.e. SDLBackend, GpuRenderer, viewport_map, platform/** with its Objective-C, and the shaders. The layout under src/ is the same, so no relative import or @embedFile inside it changes.
• backend/build.zig.zon pins SDL (fizzyedit/sdl_zig), zig-objc and zigwin32. They leave the root build.zig.zon ("one pin, held by whoever uses it"). It pins no dvui: the caller's dvui is passed in, so dvui stays pinned in sdk/ alone.
• backend/build.zig is build/exe.zig's fizzyNativeDvui and platformModule, moved:
  • backendModule(dep, dvui, opts) wires the backend under a dvui built in custom mode;
  • platformModule(dep, dvui, backend, opts) gives the platform pieces to an app on this backend or dvui's SDL3 one;
  • objcModule and win32Module are fizzy's own way to zig-objc and zigwin32.
  • addPlatformObjC(dep, root_module, opts) adds the platform's Objective-C to an app's executable, where fizzy compiled it before. The platform module does not carry it, because it calls back into the platform's Zig by name, and a compile that imports the backend without reaching that Zig fails to link. backend: health counters the native backend keeps current #309's test of the backend alone did exactly that. The backend's own macos_monitor.m stays on its module, where what it calls resolves.
  • The backend's std-only tests (unit_tests, now with backend: health counters the native backend keeps current #309's Health.zig) run under its own zig build test and still under fizzy's.
• Fizzy depends on it by path (.fizzy_backend, listed in .paths for URL consumers). The docs that said where SDL is pinned (docs/DEPENDENCIES.md, docs/MACOS_LIVE_RESIZE.md), CLAUDE.md's map and comments that named the old paths are updated.

Left for the next steps in #281: the viewports API, which is generic but still in fizzy's glue file, moves in its own PR to keep this one a pure move. After that come Popout's general half and run-dvui.

Merged with main after #309 (backend health counters), #314 (SDL pin) and #315 (Linux): their edits come along into backend/, and #314's pin is the one in backend/build.zig.zon.

SDK impact

• None. Plugins never link the backend; test-sdk-version passes unchanged.

Verified

• macOS:
  • On the merge with main: zig build 43/43; test 512/513 (1 skipped), with the backend's unit-test roots still run; test-integration 44/44 steps, 358 tests; check-web; test-sdk-version.
  • The backend's own zig build test: 46 tests.
  • An explicit -Dtarget=aarch64-macos build (43/43), which takes the path that hands the macOS SDK to the Objective-C.
  • scripts/check-examples.sh against this tree: example-plugin 28/28; example-app minimal, studio, endless 30/30, studio as data 31/31. So backend/ reaches a URL consumer.
  • CLAUDE.md's build-graph acceptance test (plugins/image, clean, ReleaseFast): no Velopack, and not the backend's SDL, in its zig-pkg.
  • The app in a sandbox profile, on the merge: Metal renderer, fizzy's SDL live-resize patch active, the native menu bar built (4 menus, 21 items: the platform's Objective-C linked and working), window and glass as before.
• Windows: cross-build (-Dtarget=x86_64-windows-gnu) 41/41; not run natively.
• Linux: CI.
• Web: check-web only (the web backend is unchanged).

🤖 Generated with Claude Code

[foxnne]

Heads-up from #314: it moves the root build.zig.zon .sdl pin from sdl_zig fizzy-1.0.3+3.4.16-8 (a02dda8) to fizzy-1.0.3+3.4.16-9 (5e41c70, hash sdl-1.0.3+3.4.16-i4QD0UwUqgA04UDxXFbmfpE1e2Oo6wwX0LJuIMG82mis): Wayland frame-inset fixes, no API change for the backend. This PR moves that pin into backend/build.zig.zon, so whichever lands second should carry the -9 URL and hash into the other's location.