Extract headless sim crate from server

Move chunk, entity, world, map, sim and load_world into a new sim library
crate together with the game rules from shared (player_action, action_delta,
delta_action, step_allowed, tile_collidable, tick constants). shared keeps
only the wire types and re-exports the rules, so server and client compile
unchanged. First step toward a local single-player build.

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

sim/src/entity.rs

@@ -0,0 +1,19 @@
+pub struct Entity {
+    pub id:      u32,
+    pub type_id: u16,   // 0 = player
+    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 {
+        Self {
+            id,
+            type_id,
+            pos,
+            hp,
+            hp_max: hp,
+        }
+    }
+}