feat(loader): per-room music field + demo rooms.yaml entries

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

Semesterprojekt/src/main/resources/world/rooms.yaml

@@ -3,6 +3,7 @@
   description: |
     A dusty kitchen. Cobwebs hang from the rafters
     and a letter lies on the table.
+  music: manor-theme.ogg
   exits:
     north: hallway
     east: cellar
@@ -55,6 +56,7 @@
   name: Dungeon
   description: |
     A cramped stone room, black as pitch. A rusty generator squats in the corner.
+  music: dungeon-drone.ogg
   dark: true
   exits:
     north: kitchen