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.toml

@@ -1,3 +1,3 @@
 [workspace]
-members = ["shared", "client", "server", "netsim"]
+members = ["sim", "shared", "client", "server", "netsim"]
 resolver = "2"