refactor(core/rendering): Surface desacoblat de Screen via Canvas (COUP-02b / COUP-03)

Surface::renderWithColorReplace usava Screen::get().getCanvasArgb() i
companys per a obtenir el buffer destí. S'introdueix una abstracció
mínima:

- source/core/rendering/canvas.hpp: struct Canvas amb pixels/width/
  height i la fàbrica activeCanvas() que retorna el canvas actiu.
- source/core/rendering/canvas.cpp: implementació que delega a Screen.

Surface ara inclou canvas.hpp en lloc de screen.hpp i crida
Rendering::activeCanvas(). L'indirecció permet, més endavant, empènyer
canvases alternatius (off-screen, efectes) sense tocar Surface; ara
mateix es comporta idèntic al codi anterior.

Build + smoke 5s xvfb OK (exit 124).

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

source/core/rendering/surface.cpp

@@ -6,8 +6,8 @@
 #include <stdexcept>
 #include <vector>
 
+#include "core/rendering/canvas.hpp"
 #include "core/rendering/gif.hpp"
-#include "core/rendering/screen.hpp"
 #include "core/resources/resource_helper.hpp"
 #include "utils/utils.hpp"
 
@@ -86,9 +86,10 @@ void Surface::render(int x, int y, const SDL_FRect* src_rect, SDL_FlipMode flip)
 }
 
 void Surface::renderWithColorReplace(int x, int y, Uint8 source_color, Uint8 target_color, const SDL_FRect* src_rect, SDL_FlipMode flip) const {
-    Uint32* dst = Screen::get().getCanvasArgb();
-    const int DST_W = Screen::get().getCanvasWidth();
-    const int DST_H = Screen::get().getCanvasHeight();
+    const auto CANVAS = Rendering::activeCanvas();
+    Uint32* dst = CANVAS.pixels;
+    const int DST_W = CANVAS.width;
+    const int DST_H = CANVAS.height;
 
     const int SX = (src_rect != nullptr) ? static_cast<int>(src_rect->x) : 0;
     const int SY = (src_rect != nullptr) ? static_cast<int>(src_rect->y) : 0;