Creatures arrive faster with more free lair room, slower as the lairs fill up

- Portal interval scaled by free beds (1 free: ~3x slower, 6: normal, 14: 2x faster, up to ~3x)
- Lair info shows the free beds and the arrival speed
- Test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lang/en.lang

@@ -503,3 +503,4 @@ camp.m19.story = Three keeps hold the last shards before the citadel. The Order
 camp.m20.title = The Grand Master
 camp.m20.story = Grand Master Valerian Brightshield waits in his citadel beyond the great river, with the last shard and the gate of the order. He has prayed, polished and prepared for a hundred years. | Destroy his gate, take back the shard and make your core whole again. I will be right behind you. Well behind you.
 camp.epilogue = The last shard sinks into the core, and the darkness breathes again. Somewhere above, the Order of the Lightwatch is writing a very long report. | Crookfinger has made tea. You are whole, master. The underworld is yours.
+lair.arrivals = {0} free beds: new creatures arrive at {1}% speed