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/entity.rs

@@ -1,15 +1,25 @@
+/// Who decides an entity's actions. The sim only schedules intents for `Npc`s; a
+/// `Player`'s intents come from outside (input) through `Sim::set_action`.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum EntityKind {
+    Player,
+    Npc,
+}
+
 pub struct Entity {
     pub id:      u32,
-    pub type_id: u16,   // 0 = player
+    pub kind:    EntityKind,
+    pub type_id: u16,   // sprite index into the entity tileset
     pub pos:     (i16, i16),
     pub hp:      u16,
     pub hp_max:  u16,
 }
 
 impl Entity {
-    pub fn new(id: u32, type_id: u16, pos: (i16, i16), hp: u16) -> Self {
+    pub fn new(id: u32, kind: EntityKind, type_id: u16, pos: (i16, i16), hp: u16) -> Self {
         Self {
             id,
+            kind,
             type_id,
             pos,
             hp,