afegit suport Emscripten/WebAssembly al build system (milestone 4)

- createSystemFolder() adaptat per Emscripten (MEMFS, sense pwd.h/unistd.h)
- initOptions() amb windowSize=1 i videoMode=0 per Emscripten
- CMakeLists.txt: SDL3 via FetchContent per Emscripten, --preload-file data
- Makefile: target wasm amb Docker (emscripten/emsdk)
- Build de Linux verificat, segueix funcionant correctament

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -30,8 +30,24 @@ if(NOT SOURCES)
 endif()
 
 # Configuración de SDL3
-find_package(SDL3 REQUIRED CONFIG REQUIRED COMPONENTS SDL3)
-message(STATUS "SDL3 encontrado: ${SDL3_INCLUDE_DIRS}")
+if(EMSCRIPTEN)
+    # En Emscripten, SDL3 se compila desde source con FetchContent
+    include(FetchContent)
+    FetchContent_Declare(
+        SDL3
+        GIT_REPOSITORY https://github.com/libsdl-org/SDL.git
+        GIT_TAG release-3.2.12
+        GIT_SHALLOW TRUE
+    )
+    set(SDL_SHARED OFF CACHE BOOL "" FORCE)
+    set(SDL_STATIC ON CACHE BOOL "" FORCE)
+    set(SDL_TEST_LIBRARY OFF CACHE BOOL "" FORCE)
+    FetchContent_MakeAvailable(SDL3)
+    message(STATUS "SDL3 compilado desde source para Emscripten")
+else()
+    find_package(SDL3 REQUIRED CONFIG REQUIRED COMPONENTS SDL3)
+    message(STATUS "SDL3 encontrado: ${SDL3_INCLUDE_DIRS}")
+endif()
 
 # Configuración común de salida de ejecutables en el directorio raíz
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR})
@@ -66,6 +82,14 @@ elseif(APPLE)
             -rpath @executable_path/../Frameworks/
         )
     endif()
+elseif(EMSCRIPTEN)
+    target_compile_definitions(${PROJECT_NAME} PRIVATE EMSCRIPTEN_BUILD)
+    target_link_options(${PROJECT_NAME} PRIVATE
+        --preload-file ${CMAKE_SOURCE_DIR}/data@/data
+        -sALLOW_MEMORY_GROWTH=1
+        -sMAX_WEBGL_VERSION=2
+    )
+    set_target_properties(${PROJECT_NAME} PROPERTIES SUFFIX ".html")
 elseif(UNIX AND NOT APPLE)
     target_compile_definitions(${PROJECT_NAME} PRIVATE LINUX_BUILD)
     target_link_options(${PROJECT_NAME} PRIVATE -Wl,--gc-sections)