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>

shared/Cargo.toml

@@ -5,3 +5,4 @@ edition = "2024"
 
 [dependencies]
 bytemuck = { version = "1.24.0", features = ["derive", "min_const_generics"] }
+sim = { path = "../sim" }