refactor: fase 1 — cleanup mecànic de baix risc (NULL→nullptr, typedef→using, explicit, enum class local)

- jdraw8.hpp: typedef → using (JD8_Surface, JD8_Palette)
- jdraw8.cpp: NULL → nullptr, C-casts → static_cast/reinterpret_cast, anon enum FadeType → enum class
- momia.cpp: NULL → nullptr
- bola/mapa/marcador/momia/engendro: explicit als constructors

Zero canvis de lògica ni ownership. Primera fase de la modernització RAII.

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

source/game/momia.cpp

@@ -40,7 +40,7 @@ Momia::Momia(JD8_Surface gfx, bool dimoni, Uint16 x, Uint16 y, Prota* sam)
     this->cur_frame = 0;
     this->o = rand() % 4;
     this->cycles_per_frame = 4;
-    this->next = NULL;
+    this->next = nullptr;
 
     if (this->dimoni) {
         if (x == 0) {
@@ -65,7 +65,7 @@ Momia::Momia(JD8_Surface gfx, bool dimoni, Uint16 x, Uint16 y, Prota* sam)
 }
 
 void Momia::clear() {
-    if (this->next != NULL) this->next->clear();
+    if (this->next != nullptr) this->next->clear();
     delete this->next;
 }
 
@@ -83,7 +83,7 @@ void Momia::draw() {
             }
         }
     }
-    if (this->next != NULL) this->next->draw();
+    if (this->next != nullptr) this->next->draw();
 }
 
 bool Momia::update() {
@@ -155,7 +155,7 @@ bool Momia::update() {
         }
     }
 
-    if (this->next != NULL) {
+    if (this->next != nullptr) {
         if (this->next->update()) {
             Momia* seguent = this->next->next;
             delete this->next;
@@ -168,7 +168,7 @@ bool Momia::update() {
 }
 
 void Momia::insertar(Momia* momia) {
-    if (this->next != NULL) {
+    if (this->next != nullptr) {
         this->next->insertar(momia);
     } else {
         this->next = momia;