fix(game): null-safe moved() + idempotent begin(); cover multi-step + go_to_room hint

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Semesterprojekt/src/main/java/thb/jeanluc/adventure/game/TutorialGuide.java

@@ -22,6 +22,7 @@ public final class TutorialGuide {
     private final CommandRegistry registry;
     private int index;
     private boolean active;
+    private boolean begun;
     private Room roomAtStepStart;
 
     public TutorialGuide(Tutorial tutorial, CommandRegistry registry) {
@@ -30,15 +31,17 @@ public final class TutorialGuide {
         this.active = !tutorial.isEmpty();
     }
 
+    /** @return true while the walkthrough is still running. */
     public boolean isActive() {
         return active;
     }
 
     /** Prints the intro and the first instruction (call once at loop start). */
     public void begin(GameContext ctx) {
-        if (!active) {
+        if (!active || begun) {
             return;
         }
+        begun = true;
         if (tutorial.intro() != null && !tutorial.intro().isBlank()) {
             ctx.getIo().write(tutorial.intro());
         }
@@ -108,7 +111,8 @@ public final class TutorialGuide {
     }
 
     private boolean moved(GameContext ctx) {
-        return ctx.getPlayer().getCurrentRoom() != roomAtStepStart;
+        return roomAtStepStart != null
+                && ctx.getPlayer().getCurrentRoom() != roomAtStepStart;
     }
 
     private boolean sameCommand(String a, String b) {