Add GPU surface magnify input

[jhodges10]

What changed

• adds GpuSurfaceInputKind.magnify at ABI ordinal 12 without renumbering existing input kinds
• forwards AppKit magnifyWithEvent: with cursor-local coordinates and incremental magnification in delta_y
• keeps widget pointer/keyboard routing exhaustive and adds platform mapping tests
• documents the macOS GPU-surface input contract

Why

Native-rendered timeline and canvas applications need first-class trackpad pinch input. Previously AppKit received magnify gestures but the Native SDK GPU-surface event API dropped them before applications could respond.

Validation

• zig build test
• zig build validate
• zig build test-appkit-gpu-magnify-input
• frontend, native, and mobile example suites
• benchmark gate
• docs typecheck and production build with pnpm 10.23.0
• zig fmt --check
• git diff --check

The optional CEF host compile was skipped because the local CEF runtime is not installed; this change is confined to the system AppKit host.

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