refactor(core/rendering): Rendering::OPAQUE_ALPHA constant (HC-02)

Surface i Screen repetien el bit d'alpha opac com a 0xFF000000U o com
(0xFFU << 24). S'afegeix inline constexpr Rendering::OPAQUE_ALPHA a
canvas.hpp i s'usa a les tres ocurrències.

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

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

source/core/rendering/canvas.hpp

@@ -10,6 +10,10 @@
 // consumidors no depenguen del cicle de vida de Screen ni del seu header.
 namespace Rendering {
 
+    // Bit alpha completament opac, ja preposicionat al canal alt del
+    // píxel ARGB8888. Qualsevol RGB | OPAQUE_ALPHA produeix el píxel final.
+    inline constexpr std::uint32_t OPAQUE_ALPHA = 0xFF000000U;
+
     struct Canvas {
         std::uint32_t* pixels{nullptr};
         int width{0};

source/core/rendering/screen.cpp

@@ -10,6 +10,7 @@
 #include <memory>
 #include <ranges>
 
+#include "core/rendering/canvas.hpp"
 #include "utils/defines.hpp"
 
 namespace {
@@ -52,7 +53,7 @@ void Screen::configure(const Config& config) {
         subsystem_initialized_ = true;
         canvas_width_ = GameCanvas::WIDTH;
         canvas_height_ = GameCanvas::HEIGHT;
-        canvas_.assign(static_cast<std::size_t>(GameCanvas::WIDTH) * static_cast<std::size_t>(GameCanvas::HEIGHT), 0xFF000000U);
+        canvas_.assign(static_cast<std::size_t>(GameCanvas::WIDTH) * static_cast<std::size_t>(GameCanvas::HEIGHT), Rendering::OPAQUE_ALPHA);
     }
     destroySdl();
     config_ = config;
@@ -214,7 +215,7 @@ void Screen::adjustWindowSize() {
 }
 
 void Screen::clear(std::uint8_t r, std::uint8_t g, std::uint8_t b) {
-    const std::uint32_t COLOR = (0xFFU << 24) | (static_cast<std::uint32_t>(r) << 16) | (static_cast<std::uint32_t>(g) << 8) | static_cast<std::uint32_t>(b);
+    const std::uint32_t COLOR = Rendering::OPAQUE_ALPHA | (static_cast<std::uint32_t>(r) << 16) | (static_cast<std::uint32_t>(g) << 8) | static_cast<std::uint32_t>(b);
     std::ranges::fill(canvas_, COLOR);
 }
 

source/core/rendering/surface.cpp

@@ -23,10 +23,10 @@ auto loadPaletteFromGif(const std::string& file_path) -> Palette {
     }
 
     Palette palette = {};
-    // Convertim RGB → ARGB amb canal alpha 0xFF.
+    // Convertim RGB → ARGB aplicant-hi el canal alpha opac constant.
     const std::size_t COUNT = std::min(rgb.size(), palette.size());
     for (std::size_t i = 0; i < COUNT; ++i) {
-        palette[i] = 0xFF000000U | rgb[i];
+        palette[i] = Rendering::OPAQUE_ALPHA | rgb[i];
     }
     return palette;
 }
@@ -60,7 +60,7 @@ Surface::Surface(const std::string& file_path) {
     std::vector<std::uint32_t> rgb = GIF::Gif::loadPalette(buffer.data());
     const std::size_t COUNT = std::min(rgb.size(), palette_.size());
     for (std::size_t i = 0; i < COUNT; ++i) {
-        palette_[i] = 0xFF000000U | rgb[i];
+        palette_[i] = Rendering::OPAQUE_ALPHA | rgb[i];
     }
 
     printWithDots("Surface : ", getFileName(file_path), "[ LOADED ]");