feat: live updates move every server onto a new build without a restart

With new binaries in place, master moves everything onto them while the
server keeps running (the dashboard's Live update, /liveupdate or SIGUSR2 to
master):

- database migrations of the new build first; a failure stops there
- UGC finishes the jobs it is running (queued rows stay pending), auth
  restarts, chat hands its teams to master for the next chat server; master
  starts the new processes and retries ones that don't come back
- once the new chat server is up (CHAT_SERVER_READY) every world connects at
  once and sends its players again (LoginSessionNotify resync, no login logged)
- every world instance is replaced with an instance migration: public worlds
  and private ones (same password) at once, properties after the old instance
  saved and froze the property (MIGRATE_PREPARE: no building, claiming or
  saving there any more), activity zones and character select once their
  players left or after a wait; empty instances just stop, zones in
  prestart_worlds get a new one first
- the dashboard restarts last and picks the status up again

Players land where they stood (position carried in CarriedPlayerState, also on
properties and Moon Base). Draining instances get no new players
(InstanceMigration::AcceptsNewPlayers) and show as "Moving players" in the
world list. The order lives in LiveUpdateMachine.h without master state and is
unit tested; master's glue is LiveUpdateCoordinator. Master itself is not
replaced. Message IDs are appended only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dGame/dUtilities/SlashCommandHandler.cpp

@@ -1714,6 +1714,15 @@ void SlashCommandHandler::Startup() {
 		.requiredLevel = eGameMasterLevel::DEVELOPER
 	});
 
+	RegisterCommand({
+		.help = "Moves the whole server onto the binaries on disk now",
+		.info = "A live update: with a new build in place, master restarts the UGC, auth and chat servers, replaces every world instance with a new one and moves its players there (with the game's Mythran maintenance warning and a short loading screen), then restarts the dashboard. Master itself keeps running. Usage: /liveupdate [start [warn seconds, 0-300] | cancel | status]",
+		.aliases = {"liveupdate"},
+		.handle = WorldMigration::LiveUpdateCommand,
+		.requiredLevel = eGameMasterLevel::OPERATOR,
+		.dashboardPermission = "server_live_update"
+	});
+
 	RegisterCommand({
 		.help = "[claim] Community challenges and live events",
 		.info = "Shows the server-wide community challenges running now, how far along they are and what you added, and the live events in this world. Also gives you the coins waiting from challenges you helped complete",