Dedicated server: park mods as server-owned objects

- maps.park_mods binds a Park Editor park mod (Mods/<folder>/parks/<key>.park.json)
  to a name and the map it was built on; maps.park_limits caps the parks and
  objects spawned at once (3 and 512 by default)
- park-mod list | add <name> [<name>...] [minutes] | remove <name> [<name>...] | clear,
  from the console, as /park-mod by admins, and by players through custom votes
- A spawned park is the layout of the server's own SteamID and epoch, sent like a
  player's: games since 1.0.0 already show it, it counts against no player's
  object limit, nobody can move or delete it, and it stays when players leave
- Objects players' games would not take (not Build Kit items, broken values) are
  skipped and logged; a missing or broken park is an error line when spawning
- Optional expiry (1-1440 min) with a warning in chat a minute before; every
  park goes on a map change and none survive a restart
- [parks] log lines name who did it: console, the admin, or "a vote"; a vote's
  refusal reaches chat
- park-mod is reserved for custom commands; README.txt documents the settings,
  commands and a park/unpark vote

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2G4APEkhHgVs59RDL3XWD

Server/server_host.cpp

@@ -871,7 +871,9 @@ void Host::change_map(std::string_view map) {
     if (!valid_map_destination(map_destination(map_setting(map))))
         throw std::invalid_argument("That map does not name a destination.");
     if (!installed_map(map)) throw std::invalid_argument("This server does not have that map.");
-    // Everything tied to the old world goes; admission and player slots stay.
+    // Everything tied to the old world goes; admission and player slots stay. Park mods only fit
+    // the map they were built on.
+    remove_parks("map change");
     for (auto &[id, guest] : guests_) {
         auto &g = *guest;
         auto old = std::exchange(g, Guest{});
@@ -1453,6 +1455,8 @@ void Host::sync_objects() {
     std::vector<Source> sources;
     for (const auto &[id, guest] : guests_)
         if (guest->handshaken && guest->world_ready) sources.push_back({id, guest->member.epoch, &guest->shared});
+    // The spawned park mods: the server's own layout, sent like a player's.
+    if (park_objects_.revision()) sources.push_back({id_, epoch_, &park_objects_});
     for (auto &[id, guest] : guests_) {
         auto &peer = *guest;
         if (!peer.handshaken || !peer.world_ready || sources.empty()) continue;
@@ -1839,6 +1843,7 @@ void Host::tick(std::uint64_t now) {
         travel_started_ = 0;
         log_("Everyone has loaded " + map_name() + ".");
     }
+    tick_parks();
     sync_objects();
     activity_.tick(now_);
     if (std::exchange(vote_recount_, false)) check_vote(false);