Add entity blocking and a wandering NPC

Occupied tiles now block movement through World::blocked, the one predicate
the sim and the pathfinder share. Entities carry an EntityKind so the sim
knows which ones to think for; Sim::think gives each NPC a random king move
one window in four. Intents execute in id order so contested tiles resolve
deterministically.

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

sim/src/world.rs

@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 use crate::chunk::{Chunk, TileFlags};
-use crate::entity::Entity;
+use crate::entity::{Entity, EntityKind};
 
 pub fn tile_to_chunk(tx: i16, ty: i16) -> (i16, i16) {
     (tx >> 5, ty >> 5)
@@ -52,14 +52,30 @@ impl World {
         chunk.set_tile(lx, ly, id);
     }
 
-    pub fn spawn_entity(&mut self, type_id: u16, pos: (i16, i16), hp: u16) -> u32 {
+    pub fn spawn_entity(&mut self, kind: EntityKind, type_id: u16, pos: (i16, i16), hp: u16) -> u32 {
         let id = self.next_entity_id;
         self.next_entity_id += 1;
-        let entity = Entity::new(id, type_id, pos, hp);
+        let entity = Entity::new(id, kind, type_id, pos, hp);
         self.add_entity(entity);
         id
     }
 
+    /// True if any entity stands on `(wx, wy)`. Goes through the chunk index, so it is
+    /// cheap enough to call per pathfinder expansion.
+    pub fn occupied(&self, wx: i16, wy: i16) -> bool {
+        let (cx, cy) = tile_to_chunk(wx, wy);
+        self.entities_in_chunk(cx, cy).iter()
+            .any(|id| self.entities[id].pos == (wx, wy))
+    }
+
+    /// The movement blocker predicate: a tile blocks if it is solid, outside the loaded
+    /// world, or occupied by an entity. Both the sim and the pathfinder plan with this.
+    pub fn blocked(&self, wx: i32, wy: i32) -> bool {
+        let Ok(wx) = i16::try_from(wx) else { return true };
+        let Ok(wy) = i16::try_from(wy) else { return true };
+        self.tile_flags(wx, wy).map_or(true, |f| f.collidable()) || self.occupied(wx, wy)
+    }
+
     pub fn add_entity(&mut self, entity: Entity) {
         let chunk_key = tile_to_chunk(entity.pos.0, entity.pos.1);
         let id = entity.id;