Add Brain enum with hunting foxes; move the A* into the sim

NPC strategy is now a Brain field on the entity (Idle, Wander, Hunt(Prey))
that Sim::think matches on, so behavior is data independent of species.
Hunt follows the first step of an A* route to the nearest prey in sight,
replanned every window; the pathfinder moves from game/ to sim/ for that.
A greedy step-toward-target was tried first and oscillated at walls. Bumps
between the same species no longer fight, and six foxes join the hens.

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

sim/src/entity.rs

@@ -6,10 +6,31 @@ pub enum EntityKind {
     Npc,
 }
 
+/// What a hunter goes after.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum Prey {
+    Player,
+    Kind(u16),
+}
+
+/// The decision strategy `Sim::think` runs for an NPC. Data, not code: it can be
+/// swapped at runtime (a hit hen may start fleeing) and serializes with the entity.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum Brain {
+    /// Never schedules anything — the player, furniture.
+    Idle,
+    /// Occasionally a random king move.
+    Wander,
+    /// Step toward the nearest prey in sight, bumping into it on contact. Wanders
+    /// when nothing is in sight.
+    Hunt(Prey),
+}
+
 pub struct Entity {
     pub id:      u32,
     pub kind:    EntityKind,
-    pub type_id: u16,   // sprite index into the entity tileset
+    pub type_id: u16,   // sprite index into the entity tileset; also the species
+    pub brain:   Brain,
     pub pos:     (i16, i16),
     pub hp:      u16,
     pub hp_max:  u16,
@@ -18,11 +39,12 @@ pub struct Entity {
 }
 
 impl Entity {
-    pub fn new(id: u32, kind: EntityKind, type_id: u16, pos: (i16, i16), hp: u16, attack: u16) -> Self {
+    pub fn new(id: u32, kind: EntityKind, type_id: u16, brain: Brain, pos: (i16, i16), hp: u16, attack: u16) -> Self {
         Self {
             id,
             kind,
             type_id,
+            brain,
             pos,
             hp,
             hp_max: hp,