fix(sdl): example builds on Zig 0.16 macOS arm64

[apotema]

Summary

The SDL backend's example/ build was failing with ~5636 errors on macOS
arm64 + Zig 0.16. Reproduces on origin/main; not caused by #226's zon
bump. Pre-existing toolchain issue, now fixed.

Root cause

Zig 0.16's bundled clang headers (lib/include/arm_vector_types.h,
arm_neon.h) use the FP8 type __mfp8 and __builtin_neon_* intrinsics
that are gated on target features (+fp8, etc.) which aren't enabled by
default for the host's apple_m* CPU. SDL2's SDL_cpuinfo.h does
#if !defined(SDL_DISABLE_ARM_NEON_H) && defined(__ARM_NEON) →
#include <arm_neon.h>, transitively pulling Zig's broken-for-this-CPU
header into the @cImport translation unit.

First 5 errors from the broken build:

arm_vector_types.h:20:9: error: type specifier missing, defaults to 'int'
  typedef __mfp8 mfloat8_t;
arm_vector_types.h:20:16: error: expected ';', found 'an identifier'
arm_vector_types.h:93:46: error: type specifier missing ...
  typedef __attribute__((neon_vector_type(8))) mfloat8_t mfloat8x8_t;
arm_vector_types.h:171:3: error: unknown type name 'mfloat8x8_t'
arm_neon.h:246:43: error: use of unknown builtin '__builtin_neon_vbfdotq_f32'

Fix

Define SDL_DISABLE_ARM_NEON_H (SDL's official escape hatch) at every
@cImport site for SDL2 in the backend:

• backends/sdl/src/sdl.zig
• backends/sdl/src/audio.zig (separate cImport for SDL_mixer)

We don't call NEON intrinsics from Zig — SDL still uses NEON internally
in its precompiled library — so disabling the transitive header include
is safe and surgical.

Verification (macOS arm64, Zig 0.16.0)

• cd backends/sdl/example && zig build — green (was 5636 errors)
• cd backends/sdl && zig build — green
• cd backends/sdl && zig build test — green
• cd backends/sdl && zig build -Dtarget=x86_64-linux-gnu — green
• cd backends/sdl && zig build -Dtarget=x86_64-windows-gnu — green

(Example cross-compile to Linux still fails, but on a pre-existing
unconditional linkFramework("Cocoa") in example/build.zig — unrelated
to this fix.)

Related / upstream

The underlying issue — Zig 0.16 shipping FP8-aware arm_neon.h without
defaulting +fp8 for apple_m* CPUs — is arguably a Zig stdlib bug worth
filing upstream. Any C library that includes <arm_neon.h> from a public
header (not just SDL2) will hit this. Workaround here is SDL-specific;
a generalized fix would live in ziglang/zig.

Test plan

• sdl example builds on macOS arm64
• sdl backend builds + tests pass on macOS arm64
• sdl backend cross-compiles to linux + windows
• CI confirms green on all platforms

[cursor]

PR Summary

Low Risk
Compile-time C import workaround only; no runtime or API behavior changes, and NEON is not used from Zig code.

Overview
Fixes macOS arm64 + Zig 0.16 SDL backend and example builds that were failing with thousands of C translation errors when SDL pulled in <arm_neon.h> via SDL_cpuinfo.h.

Both SDL @cImport blocks now define SDL_DISABLE_ARM_NEON_H before including headers: the shared import in sdl.zig and the separate SDL_mixer import in audio.zig. That stops the broken FP8/NEON header chain from entering Zig’s @cImport units while SDL still uses NEON in its linked library. Comments in sdl.zig (and a short pointer in audio.zig) document the toolchain mismatch and why the define is safe.

^{Reviewed by Cursor Bugbot for commit 3be0098. Bugbot is set up for automated code reviews on this repo. Configure here.}

[gemini-code-assist]

Code Review

This pull request defines SDL_DISABLE_ARM_NEON_H as 1 before importing SDL2 headers in backends/sdl/src/audio.zig and backends/sdl/src/sdl.zig. This change prevents compilation errors on macOS arm64 with Zig 0.16 caused by a transitive inclusion of <arm_neon.h> and a mismatch with the FP8 type. There are no review comments, and I have no additional feedback to provide.