feat(grid): graella verda fosca de fons al playfield (16x8)

source/core/graphics/playfield_grid.cpp

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+// playfield_grid.cpp - Implementació de la grilla de fons
+// © 2026 JailDesigner
+
+#include "core/graphics/playfield_grid.hpp"
+
+#include "core/defaults.hpp"
+#include "core/rendering/line_renderer.hpp"
+
+namespace Graphics {
+
+    PlayfieldGrid::PlayfieldGrid(Rendering::Renderer* renderer)
+        : renderer_(renderer) {}
+
+    void PlayfieldGrid::draw() const {
+        const SDL_FRect& zona = Defaults::Zones::PLAYAREA;
+        const float CELL_W = zona.w / static_cast<float>(Defaults::Grid::COLUMNS);
+        const float CELL_H = zona.h / static_cast<float>(Defaults::Grid::ROWS);
+
+        const int X1 = static_cast<int>(zona.x);
+        const int Y1 = static_cast<int>(zona.y);
+        const int X2 = static_cast<int>(zona.x + zona.w);
+        const int Y2 = static_cast<int>(zona.y + zona.h);
+
+        // Línies verticals interiors (i=1..COLUMNS-1: no repetim el border)
+        for (int i = 1; i < Defaults::Grid::COLUMNS; i++) {
+            const int X = static_cast<int>(zona.x + (static_cast<float>(i) * CELL_W));
+            Rendering::linea(renderer_, X, Y1, X, Y2, Defaults::Grid::BRIGHTNESS);
+        }
+
+        // Línies horitzontals interiors (j=1..ROWS-1: no repetim el border)
+        for (int j = 1; j < Defaults::Grid::ROWS; j++) {
+            const int Y = static_cast<int>(zona.y + (static_cast<float>(j) * CELL_H));
+            Rendering::linea(renderer_, X1, Y, X2, Y, Defaults::Grid::BRIGHTNESS);
+        }
+    }
+
+}  // namespace Graphics

source/core/graphics/playfield_grid.hpp

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+// playfield_grid.hpp - Grilla de fons del playfield (línies verdes fosques)
+// © 2026 JailDesigner
+
+#pragma once
+
+#include "core/rendering/render_context.hpp"
+
+namespace Graphics {
+
+    // Pinta una graella interior dins de Defaults::Zones::PLAYAREA.
+    // Cel·les quadrades; el nombre de columnes és fix (Defaults::Grid::COLUMNS),
+    // les files es deriven del que cap dins de l'altura.
+    // Color: oscil·lador global (mateix to que el border); brightness fixa al 25%.
+    class PlayfieldGrid {
+       public:
+        explicit PlayfieldGrid(Rendering::Renderer* renderer);
+        void draw() const;
+
+       private:
+        Rendering::Renderer* renderer_;
+    };
+
+}  // namespace Graphics