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>

Cargo.lock

@@ -1477,6 +1477,7 @@ version = "0.1.0"
 dependencies = [
  "bytemuck",
  "shared",
+ "sim",
 ]
 
 [[package]]
@@ -1484,6 +1485,7 @@ name = "shared"
 version = "0.1.0"
 dependencies = [
  "bytemuck",
+ "sim",
 ]
 
 [[package]]
@@ -1492,6 +1494,10 @@ version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0fda2ff0d084019ba4d7c6f371c95d8fd75ce3524c3cb8fb653a3023f6323e64"
 
+[[package]]
+name = "sim"
+version = "0.1.0"
+
 [[package]]
 name = "simd_cesu8"
 version = "1.1.1"