build: hacer Vulkan SDK opcional usando SPIR-V pre-compilados

Si glslc no está disponible, CMake usa los headers en shaders/precompiled/
en lugar de fallar. Los SPIR-V son portátiles entre Windows y Linux, por
lo que el build funciona sin instalar Vulkan SDK.

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

CMakeLists.txt

@@ -28,9 +28,9 @@ endif()
 if(NOT APPLE)
     find_program(GLSLC glslc HINTS "$ENV{VULKAN_SDK}/bin" "$ENV{VULKAN_SDK}/Bin")
     if(NOT GLSLC)
-        message(FATAL_ERROR "glslc not found. Install the Vulkan SDK and ensure glslc is on PATH.")
-    endif()
-
+        message(STATUS "glslc not found — using precompiled SPIR-V headers from shaders/precompiled/")
+        set(SHADER_GEN_DIR "${CMAKE_SOURCE_DIR}/shaders/precompiled")
+    else()
         set(SHADER_SRC_DIR "${CMAKE_SOURCE_DIR}/shaders")
         set(SHADER_GEN_DIR "${CMAKE_BINARY_DIR}/generated_shaders")
         file(MAKE_DIRECTORY "${SHADER_GEN_DIR}")
@@ -65,6 +65,7 @@ if(NOT APPLE)
 
         add_custom_target(shaders ALL DEPENDS ${SPIRV_HEADERS})
     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)
@@ -103,7 +104,9 @@ set_target_properties(${PROJECT_NAME} PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAK
 target_link_libraries(${PROJECT_NAME} ${LINK_LIBS})
 
 if(NOT APPLE)
+    if(GLSLC)
         add_dependencies(${PROJECT_NAME} shaders)
+    endif()
     target_include_directories(${PROJECT_NAME} PRIVATE "${SHADER_GEN_DIR}")
 endif()