style: aplicar checks modernize-* (215 fixes)

Cambios aplicados:
- [[nodiscard]] añadido a funciones que retornan valores
- .starts_with() en lugar de .find() == 0
- Inicializadores designados {.x=0, .y=0}
- auto en castings obvios
- = default para constructores triviales
- Funciones deleted movidas a public
- std::numbers::pi_v<float> (C++20)

Checks excluidos:
- use-trailing-return-type: Estilo controversial
- avoid-c-arrays: Arrays C aceptables en ciertos contextos

source/core/graphics/shape_loader.cpp

@@ -24,7 +24,7 @@ std::shared_ptr<Shape> ShapeLoader::load(const std::string& filename) {
     // Normalize path: "ship.shp" → "shapes/ship.shp"
     //                 "logo/letra_j.shp" → "shapes/logo/letra_j.shp"
     std::string normalized = filename;
-    if (normalized.find("shapes/") != 0) {
+    if (!normalized.starts_with("shapes/")) {
         // Doesn't start with "shapes/", so add it
         normalized = "shapes/" + normalized;
     }
@@ -75,7 +75,7 @@ std::string ShapeLoader::resolve_path(const std::string& filename) {
     }
 
     // Si ja conté el prefix base_path, usar-lo directament
-    if (filename.find(base_path_) == 0) {
+    if (filename.starts_with(base_path_)) {
         return filename;
     }