Fase 9b: extraer ContinueSystem de GameScene

GameScene::actualitzar_continue, processar_input_continue y el
helper check_and_apply_continue_timeout (3 funciones, ~140 LOC) salen
a Systems::ContinueScreen en source/game/systems/continue_system.{hpp,cpp}.

API:
- struct Systems::ContinueScreen::Context: agrupa el estado mutable
  (state, counter, tick_timer, continues_used, game_over_timer,
  lives/score/hit_timer arrays, ships, match_config) y un callback
  get_spawn_point inyectado por GameScene.
- update(ctx, dt): avanza countdown automatico y transiciona a
  GAME_OVER si timeout.
- processInput(ctx): START revive jugador(es), THRUST/SHOOT acelera
  countdown.

Helpers privados (revivePlayer, checkAndApplyTimeout) en anonymous
namespace del .cpp para evitar contaminar el header.

GameOverState ahora con underlying type explicito (uint8_t) para
permitir forward-declaration limpia en continue_system.hpp.

dibuixar_continue y unir_jugador se quedan en GameScene (render y
gameplay normal, no parte del state machine).

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

source/game/scenes/game_scene.hpp

@@ -25,7 +25,7 @@
 #include "game/stage_system/stage_manager.hpp"
 
 // Game over state machine
-enum class GameOverState {
+enum class GameOverState : uint8_t {
     NONE,      // Normal gameplay
     CONTINUE,  // Continue countdown screen (9→0)
     GAME_OVER  // Final game over (returning to title)
@@ -89,9 +89,6 @@ class GameScene {
 
         // [NEW] Continue & Join system
         void unir_jugador(uint8_t player_id);         // Join inactive player mid-game
-        void processar_input_continue();              // Handle input during continue screen
-        void actualitzar_continue(float delta_time);  // Update continue countdown
-        void check_and_apply_continue_timeout();      // Check if continue timed out and transition to GAME_OVER
         void dibuixar_continue();                     // Draw continue screen
 
         // [NEW] Stage system helpers