fix(items): make scenery items fixed so they cannot be taken

Nothing stopped the player from picking up scenery. The front door is an
ordinary switchable item in the foyer whose on-transition sets the 'left_manor'
flag that every ending keys off, so it could be carried into the cellar and used
there to end the game two floors underground. The chapel shrine could likewise
be carried out and the locket/photograph ritual performed anywhere.

Item gains a 'fixed' flag; TakeCommand refuses fixed items and leaves them in
the room. Fixed items stay usable in place, so all recipes still work.

Marked fixed: front_door, shrine, generator, valve, portrait — their
descriptions already claimed they were bolted down.

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

@@ -40,6 +40,7 @@
 - type: plain
   id: valve
   name: Drainage Valve
+  fixed: true
   description: |
     A rusted drainage valve set into the cellar wall. Its wheel is missing —
     it is part of the house, not something you can carry off.
@@ -52,6 +53,7 @@
 - type: plain
   id: generator
   name: Generator
+  fixed: true
   description: |
     A rusty generator with an empty fuse socket. It is bolted to the floor —
     you will have to work on it where it stands.
@@ -71,6 +73,7 @@
 - type: plain
   id: shrine
   name: Stone Shrine
+  fixed: true
   description: |
     A small stone shrine with a shallow offering niche. It is mortared into
     the chapel floor — a place to leave something, not to take it.
@@ -80,6 +83,7 @@
 - type: switchable
   id: front_door
   name: Front Door
+  fixed: true
   description: The heavy front door. It would let you leave the manor for good.
   initialState: false
   onText: |
@@ -113,6 +117,7 @@
 - type: readable
   id: portrait
   name: Family Portrait
+  fixed: true
   description: A faded oil portrait above the dining-room hearth.
   readText: |
     A stern family poses before the manor. A small brass plate reads only: