feat(world): add the Lost Twins optional escort side-quest

Two separated brothers, Marlon (conservatory) and Malte (boiler room), can be
reunited as optional content. Carry each twin's marble to the other as proof
he's alive and that twin follows you; lead both to the foyer and they escape
together, granting a Brass Compass keepsake and unlocking a new top-priority
"No One Left Behind" ending. Purely optional — no main win condition depends
on it.

The new EscortEngine drives following and the foyer reunion from flags alone,
so save/load needs no format change (it reconciles follower placement on the
first tick after loading). Adds Room.removeNpc, one tick call in the game loop,
and a follower note appended by LookCommand. descriptionStates swap the twins'
home-room prose once they leave.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -74,11 +74,18 @@
   description: |
     A glass-roofed room choked with dead ferns. Rusted plumbing lines the
     walls, and a heavy iron valve wheel has been left on a potting bench.
+    A frightened young man cowers among the planters, a blue marble in his fist.
   music: manor-theme.ogg
   exits:
     south: dining_room
-  items: [valve_wheel]
-  npcs: []
+  items: [valve_wheel, marlon_keepsake]
+  npcs: [marlon]
+  descriptionStates:
+    - when: [{ flag: marlon_following }]
+      text: |
+        A glass-roofed room choked with dead ferns. Rusted plumbing lines the
+        walls, and a heavy iron valve wheel has been left on a potting bench.
+        The corner where the frightened young man crouched is empty now.
 
 - id: grand_staircase
   name: Grand Staircase
@@ -135,13 +142,20 @@
   name: Boiler Room
   description: |
     A cramped stone room behind the furnace. A rusty generator squats in the
-    corner, an empty fuse socket gaping in its housing.
+    corner, an empty fuse socket gaping in its housing. A shivering young man
+    huddles against the cold iron, a red marble clenched in his hand.
   music: cellar-drone.ogg
   dark: true
   exits:
     east: cellar_stairs
-  items: [generator]
-  npcs: []
+  items: [generator, malte_keepsake]
+  npcs: [malte]
+  descriptionStates:
+    - when: [{ flag: malte_following }]
+      text: |
+        A cramped stone room behind the furnace. A rusty generator squats in
+        the corner, an empty fuse socket gaping in its housing. The cold spot
+        by the iron where the shivering man huddled is empty now.
 
 # ── Upper floor (reachable only once the power is on) ───────────────────────