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>

CMakeLists.txt

@@ -94,6 +94,33 @@ if(NOT APPLE)
     endif()
 endif()
 
+# macOS: transpile .spv → .msl for GpuShaderPreset
+if(APPLE)
+    find_program(SPIRV_CROSS spirv-cross)
+    if(SPIRV_CROSS)
+        file(GLOB_RECURSE DATA_SHADER_SPVS
+            "${CMAKE_SOURCE_DIR}/data/shaders/*.vert.spv"
+            "${CMAKE_SOURCE_DIR}/data/shaders/*.frag.spv"
+        )
+        set(DATA_SHADER_MSLS)
+        foreach(SPV_FILE ${DATA_SHADER_SPVS})
+            set(MSL_FILE "${SPV_FILE}.msl")
+            add_custom_command(
+                OUTPUT "${MSL_FILE}"
+                COMMAND "${SPIRV_CROSS}" --msl --output "${MSL_FILE}" "${SPV_FILE}"
+                DEPENDS "${SPV_FILE}"
+                COMMENT "Transpiling ${SPV_FILE} to MSL"
+            )
+            list(APPEND DATA_SHADER_MSLS "${MSL_FILE}")
+        endforeach()
+        if(DATA_SHADER_MSLS)
+            add_custom_target(data_shader_msls ALL DEPENDS ${DATA_SHADER_MSLS})
+        endif()
+    else()
+        message(STATUS "spirv-cross not found — GpuShaderPreset will not work on macOS (run: brew install spirv-cross)")
+    endif()
+endif()
+
 # Archivos fuente (excluir main_old.cpp)
 file(GLOB SOURCE_FILES source/*.cpp source/external/*.cpp source/boids_mgr/*.cpp source/gpu/*.cpp source/input/*.cpp source/scene/*.cpp source/shapes/*.cpp source/shapes_mgr/*.cpp source/state/*.cpp source/themes/*.cpp source/text/*.cpp source/ui/*.cpp)
 list(REMOVE_ITEM SOURCE_FILES "${CMAKE_SOURCE_DIR}/source/main_old.cpp")
@@ -138,6 +165,10 @@ if(NOT APPLE)
     endif()
 endif()
 
+if(APPLE AND TARGET data_shader_msls)
+    add_dependencies(${PROJECT_NAME} data_shader_msls)
+endif()
+
 # Tool: pack_resources
 add_executable(pack_resources tools/pack_resources.cpp source/resource_pack.cpp)
 target_include_directories(pack_resources PRIVATE ${CMAKE_SOURCE_DIR}/source)