feat(shaders): sistema de shaders runtime amb presets externs

- Afegir GpuShaderPreset i ShaderManager per carregar shaders des de data/shaders/
- Implementar preset ntsc-md-rainbows (2 passos: encode + decode MAME NTSC)
- Render loop multi-pass per shaders externs (targets intermedis R16G16B16A16_FLOAT)
- cycleShader(): cicla OFF→PostFX natius→shaders externs amb tecla X
- --shader <nom> per arrancar directament amb un preset extern
- CMake auto-descubreix i compila data/shaders/**/*.vert/.frag → .spv
- HUD F1 mostra 'Shader: <nom>' quan hi ha shader extern actiu

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

CMakeLists.txt

@@ -64,6 +64,34 @@ if(NOT APPLE)
     endforeach()
 
     add_custom_target(shaders ALL DEPENDS ${SPIRV_HEADERS})
+
+    # External runtime shaders: auto-discover and compile data/shaders/**/*.vert/*.frag
+    # Output: <source>.spv alongside each source file (loaded at runtime by GpuShaderPreset)
+    file(GLOB_RECURSE DATA_SHADERS
+        "${CMAKE_SOURCE_DIR}/data/shaders/**/*.vert"
+        "${CMAKE_SOURCE_DIR}/data/shaders/**/*.frag")
+
+    set(DATA_SHADER_SPVS)
+    foreach(SHADER_FILE ${DATA_SHADERS})
+        get_filename_component(SHADER_EXT "${SHADER_FILE}" EXT)
+        if(SHADER_EXT STREQUAL ".vert")
+            set(STAGE_FLAG "-fshader-stage=vertex")
+        else()
+            set(STAGE_FLAG "-fshader-stage=fragment")
+        endif()
+        set(SPV_FILE "${SHADER_FILE}.spv")
+        add_custom_command(
+            OUTPUT "${SPV_FILE}"
+            COMMAND "${GLSLC}" ${STAGE_FLAG} -o "${SPV_FILE}" "${SHADER_FILE}"
+            DEPENDS "${SHADER_FILE}"
+            COMMENT "Compiling ${SHADER_FILE}"
+        )
+        list(APPEND DATA_SHADER_SPVS "${SPV_FILE}")
+    endforeach()
+
+    if(DATA_SHADER_SPVS)
+        add_custom_target(data_shaders ALL DEPENDS ${DATA_SHADER_SPVS})
+    endif()
 endif()
 
 # Archivos fuente (excluir main_old.cpp)
@@ -105,6 +133,9 @@ target_link_libraries(${PROJECT_NAME} ${LINK_LIBS})
 if(NOT APPLE)
     add_dependencies(${PROJECT_NAME} shaders)
     target_include_directories(${PROJECT_NAME} PRIVATE "${SHADER_GEN_DIR}")
+    if(TARGET data_shaders)
+        add_dependencies(${PROJECT_NAME} data_shaders)
+    endif()
 endif()
 
 # Tool: pack_resources