feat(macos): suport Metal per GpuShaderPreset via spirv-cross

- CMakeLists: bloc if(APPLE) que transpila .spv → .spv.msl amb spirv-cross
- gpu_shader_preset: carrega MSL (main0) a macOS, SPIR-V (main) a la resta
- Afegeix null-terminator als buffers MSL (SDL3 els tracta com C-string)
- README: secció de dependències de shaders per plataforma (Vulkan SDK, spirv-cross)
- Inclou .spv.msl generats per ntsc-md-rainbows

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

data/shaders/ntsc-md-rainbows/pass0_encode.vert.spv.msl

@@ -0,0 +1,63 @@
+#pragma clang diagnostic ignored "-Wmissing-prototypes"
+#pragma clang diagnostic ignored "-Wmissing-braces"
+
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+template<typename T, size_t Num>
+struct spvUnsafeArray
+{
+    T elements[Num ? Num : 1];
+    
+    thread T& operator [] (size_t pos) thread
+    {
+        return elements[pos];
+    }
+    constexpr const thread T& operator [] (size_t pos) const thread
+    {
+        return elements[pos];
+    }
+    
+    device T& operator [] (size_t pos) device
+    {
+        return elements[pos];
+    }
+    constexpr const device T& operator [] (size_t pos) const device
+    {
+        return elements[pos];
+    }
+    
+    constexpr const constant T& operator [] (size_t pos) const constant
+    {
+        return elements[pos];
+    }
+    
+    threadgroup T& operator [] (size_t pos) threadgroup
+    {
+        return elements[pos];
+    }
+    constexpr const threadgroup T& operator [] (size_t pos) const threadgroup
+    {
+        return elements[pos];
+    }
+};
+
+constant spvUnsafeArray<float2, 3> _18 = spvUnsafeArray<float2, 3>({ float2(-1.0), float2(3.0, -1.0), float2(-1.0, 3.0) });
+constant spvUnsafeArray<float2, 3> _26 = spvUnsafeArray<float2, 3>({ float2(0.0, 1.0), float2(2.0, 1.0), float2(0.0, -1.0) });
+
+struct main0_out
+{
+    float2 v_uv [[user(locn0)]];
+    float4 gl_Position [[position]];
+};
+
+vertex main0_out main0(uint gl_VertexIndex [[vertex_id]])
+{
+    main0_out out = {};
+    out.gl_Position = float4(_18[int(gl_VertexIndex)], 0.0, 1.0);
+    out.v_uv = _26[int(gl_VertexIndex)];
+    return out;
+}
+