Fase 7a: infraestructura SDL3 GPU (dormida, sin tocar runtime)

Preparacion del pipeline GPU. Codigo nuevo aislado en
core/rendering/gpu/; el runtime sigue usando SDL_Renderer hasta
Fase 7b. Tras 7a el juego sigue funcionando identico.

Shaders (shaders/):
- line.vert.glsl: vertex shader, transforma de pixeles logicos a NDC
  via uniform buffer LineUniforms{viewport_w, viewport_h}.
- line.frag.glsl: pinta el color RGBA interpolado.

Build:
- CMakeLists.txt: step nuevo que compila *.glsl a build/shaders/*.spv
  con glslc. ALL depende del target 'shaders' para incluirlo en cada
  build. Falla en cmake config si glslc no esta instalado.

Wrappers C++ (source/core/rendering/gpu/):
- gpu_device.hpp/cpp: GpuDevice, claim del window, loadShader desde
  .spv. Backends solicitados: Vulkan + Metal (sin DirectX).
- gpu_line_pipeline.hpp/cpp: GpuLinePipeline. Vertex layout
  (vec2 pos + vec4 color), primitive TRIANGLELIST (lineas como
  quads), alpha blending estandar, sin culling ni depth.
- gpu_frame_renderer.hpp/cpp: GpuFrameRenderer, API alto nivel:
  beginFrame / pushLine / endFrame. Extrusion perpendicular en CPU
  por linea (thickness libre por linea). Un draw call por frame
  con vertex+index buffers transitorios.

Plan: 7b swap del SDL_Renderer al GpuFrameRenderer en SDLManager.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -135,6 +135,46 @@ add_custom_command(
 add_custom_target(resource_pack ALL DEPENDS ${RESOURCE_PACK})
 add_dependencies(${PROJECT_NAME} resource_pack)
 
+# --- COMPILACIÓ DE SHADERS GLSL → SPIR-V ---
+# Compila tots els shaders .glsl a SPIR-V (Vulkan/Linux/Windows).
+# macOS necessitarà MSL en el futur (Metal) — es generen amb spirv-cross
+# o glslang amb target distint en una etapa posterior.
+# Sortida: build/shaders/*.spv
+find_program(GLSLC_EXE NAMES glslc HINTS ${Vulkan_GLSLC_EXECUTABLE})
+if(GLSLC_EXE)
+    file(GLOB SHADER_SOURCES CONFIGURE_DEPENDS "${CMAKE_SOURCE_DIR}/shaders/*.glsl")
+    set(COMPILED_SHADERS "")
+    foreach(SHADER ${SHADER_SOURCES})
+        get_filename_component(SHADER_NAME ${SHADER} NAME)
+        # Detectar stage del nom: line.vert.glsl → vert, line.frag.glsl → frag
+        if(SHADER_NAME MATCHES "\\.vert\\.glsl$")
+            set(SHADER_STAGE "vert")
+            string(REPLACE ".glsl" ".spv" SPV_NAME ${SHADER_NAME})
+        elseif(SHADER_NAME MATCHES "\\.frag\\.glsl$")
+            set(SHADER_STAGE "frag")
+            string(REPLACE ".glsl" ".spv" SPV_NAME ${SHADER_NAME})
+        else()
+            message(WARNING "Shader sense stage detectat: ${SHADER_NAME} (esperat .vert.glsl o .frag.glsl)")
+            continue()
+        endif()
+        set(SPV_OUTPUT "${CMAKE_BINARY_DIR}/shaders/${SPV_NAME}")
+        add_custom_command(
+            OUTPUT ${SPV_OUTPUT}
+            COMMAND ${CMAKE_COMMAND} -E make_directory "${CMAKE_BINARY_DIR}/shaders"
+            COMMAND ${GLSLC_EXE} -fshader-stage=${SHADER_STAGE} -O ${SHADER} -o ${SPV_OUTPUT}
+            DEPENDS ${SHADER}
+            COMMENT "Compilant shader ${SHADER_NAME} → ${SPV_NAME}"
+            VERBATIM
+        )
+        list(APPEND COMPILED_SHADERS ${SPV_OUTPUT})
+    endforeach()
+    add_custom_target(shaders ALL DEPENDS ${COMPILED_SHADERS})
+    add_dependencies(${PROJECT_NAME} shaders)
+    message(STATUS "Shaders trobats: ${SHADER_SOURCES}")
+else()
+    message(FATAL_ERROR "glslc no trobat: instal·la 'shaderc' o 'vulkan-sdk' per compilar shaders SPIR-V")
+endif()
+
 # --- STATIC ANALYSIS / FORMAT TARGETS ---
 find_program(CLANG_TIDY_EXE NAMES clang-tidy)
 find_program(CLANG_FORMAT_EXE NAMES clang-format)