Organic construction: buildings from need, not quotas

Removes every predetermined construction trigger (shelters-per-capita
cap, granary-at-five-shelters milestone). Buildings now emerge from
lived need:

- Shelters have real bed capacity (3 + modules). A full house turns you
  away; three nights sleeping outside and a person gathers stone and
  raises a roof where they actually live. When nobody sleeps outside,
  nobody builds. Ruined shelters spill their sleepers, so settlements
  can migrate over time.
- The first granary is founded by memory: someone who has truly starved
  stockpiles food once the land gives again and builds storage where the
  harvest is. A second granary appears only when the first fills.
- Emergent consequence: freeriders compete for beds but never build,
  so scarce housing squeezes them into the open. The village never
  decides to punish them; the housing supply does.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/GameManager.cs

@@ -117,6 +117,16 @@ public partial class GameManager : Node2D
         }
     }
 
+    /// <summary>How many villagers are currently sleeping at this shelter.</summary>
+    public int RestingOccupancy(Building b)
+    {
+        int count = 0;
+        foreach (var npc in Npcs)
+            if (npc.State == NpcState.Resting && npc.Position.DistanceTo(b.Site) <= 2.5f)
+                count++;
+        return count;
+    }
+
     public int TownPopulation(string town)
     {
         int count = 0;