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/endings.yaml

@@ -2,6 +2,17 @@
 # specific (best) ending is listed first. ghost_banished implies power_on
 # implies the upper floor was reached, so #1 needs only left_manor + ghost_banished.
 
+# Best possible end: spirit laid to rest AND the lost twins led out (optional).
+- id: no_one_left_behind
+  title: "No One Left Behind"
+  victory: true
+  when: [{ flag: left_manor }, { flag: ghost_banished }, { flag: twins_reunited }]
+  text: |
+    You step into the night, and you are not alone. Marlon and Malte wait at
+    the gate, together, breathing the cold free air. Behind you the manor is
+    utterly still. You laid a spirit to rest and brought two brothers home —
+    there is no better way out of that house than this one.
+
 - id: spirit_at_rest
   title: "Spirit at Rest"
   victory: true

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

@@ -122,3 +122,27 @@
   id: shovel
   name: Shovel
   description: A rusty shovel. Sturdy enough to dig, though nothing here needs digging.
+
+# ── The lost twins (optional escort side-quest) ──────────────────────────────
+
+- type: plain
+  id: marlon_keepsake
+  name: Blue Marble
+  description: |
+    A child's blue glass marble, worn smooth from years in a pocket. Marlon's.
+    Somewhere its red twin is being clutched just as tightly.
+
+- type: plain
+  id: malte_keepsake
+  name: Red Marble
+  description: |
+    A child's red glass marble, the mate to a blue one. Malte's — the only
+    thing he kept hold of when the dark took him.
+
+- type: readable
+  id: brass_compass
+  name: Brass Compass
+  description: A small brass compass, pressed on you by two very grateful brothers.
+  readText: |
+    Engraved on the back, in a child's scratchy hand: "For the one who led us
+    out. — M & M." The needle, you notice, always settles toward the door.

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 }

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

@@ -16,3 +16,13 @@
   stages:
     - objective: "Lay the restless spirit to rest in the chapel."
       completion: [{ flag: ghost_banished }]
+
+# Optional side-quest: started when you first recruit a twin (see npcs.yaml).
+- id: reunite_twins
+  title: "The Lost Twins"
+  autoStart: false
+  stages:
+    - objective: "Reunite Marlon and Malte — lead them both to the entrance."
+      completion: [{ flag: twins_reunited }]
+  onComplete:
+    - { say: "The brothers are together again, and gone into the night. You didn't have to help them. You're glad you did." }

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) ───────────────────────