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/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.