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/npcs.yaml

@@ -27,3 +27,54 @@
       consumes: letter
       effects:
         - { setFlag: heard_tale }
+
+# ── The lost twins (optional escort side-quest) ──────────────────────────────
+# Marlon (conservatory) and Malte (boiler room) were separated in the manor.
+# Give each the other's marble as proof his brother lives, and he follows you;
+# lead both to the foyer and they escape together. See EscortEngine.
+
+- id: marlon
+  name: Marlon
+  description: |
+    A frightened young man pressed into the corner, knuckles white around a
+    blue glass marble. He has his brother's face — minus the brother.
+  greeting: |
+    "Please — have you seen Malte? My twin. We were exploring and the dark
+    just... took him. I can't leave this house without him. I can't."
+  dialogue:
+    - when: [{ hasItem: malte_keepsake }]
+      text: |
+        "That red marble — that's MALTE'S! He's alive? Here, quick: give it to
+        me so I know it's real. Then I'll follow you to him."
+  reactions:
+    - onReceive: malte_keepsake
+      response: |
+        "His marble. His actual marble. Then he's alive — and you've seen him.
+        Lead the way. I'm right behind you, I won't lose him again."
+      consumes: malte_keepsake
+      effects:
+        - { setFlag: marlon_following }
+        - { startQuest: reunite_twins }
+
+- id: malte
+  name: Malte
+  description: |
+    A shivering young man clutching a red glass marble in the furnace-dark.
+    The image of his brother, alone.
+  greeting: |
+    "Who's there? Marlon? No... Please, have you seen him — my twin, Marlon?
+    We got split up and I can't find my way back in this dark."
+  dialogue:
+    - when: [{ hasItem: marlon_keepsake }]
+      text: |
+        "A blue marble — Marlon's! You've found him! Give it here so I know
+        it's real, then take me to him, please, before I lose my nerve."
+  reactions:
+    - onReceive: marlon_keepsake
+      response: |
+        "Marlon's marble. He's really here. Don't let go of me — wherever
+        you're going, I'm following. Take me to my brother."
+      consumes: marlon_keepsake
+      effects:
+        - { setFlag: malte_following }
+        - { startQuest: reunite_twins }