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/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 ]");