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>

src/game.rs

@@ -834,7 +834,7 @@ impl Game {
         if owned > 0 {
             self.portal_timer -= dt;
             if self.portal_timer <= 0.0 {
-                self.portal_timer = (20.0 + self.rng.f() * 14.0) / (owned as f32).sqrt();
+                self.portal_timer = (20.0 + self.rng.f() * 14.0) / (owned as f32).sqrt() * self.lair_factor();
                 self.try_portal();
             }
         }
@@ -938,6 +938,15 @@ impl Game {
         }
     }
 
+    // free beds in the lairs: much room lets creatures arrive faster, little room slower
+    pub fn free_beds(&self) -> usize {
+        self.counts[1].min(30).saturating_sub(self.creature_count())
+    }
+
+    pub fn lair_factor(&self) -> f32 {
+        (8.0 / (self.free_beds() as f32 + 2.0)).clamp(0.35, 3.0)
+    }
+
     fn try_portal(&mut self) {
         let creatures = self.creature_count();
         let lair = self.counts[1];
@@ -4052,6 +4061,23 @@ mod tests {
         assert!(g.gate_hp < GATE_HP, "the gate is shot at");
     }
 
+    #[test]
+    fn more_lair_room_brings_creatures_faster() {
+        let mut g = Game::new(3, 64, 64);
+        g.counts[1] = 4;
+        let tight = g.lair_factor();
+        g.counts[1] = 12;
+        let normal = g.lair_factor();
+        g.counts[1] = 30;
+        let roomy = g.lair_factor();
+        assert!(tight > normal && normal > roomy, "{} {} {}", tight, normal, roomy);
+        // creatures fill the lair: arrivals slow down again
+        for _ in 0..20 {
+            g.spawn(EK::Kobold, false, 10.0, 10.0, 1);
+        }
+        assert!(g.lair_factor() > roomy);
+    }
+
     #[test]
     fn vaults_are_claimed_and_looted() {
         let (mut g, _) = fort_game(|x, y| crate::fortress::FortSpec { vaults: 2, vault_value: 1800, ..spec(x, y) });