Add arrivals (#14755)

* Arrivals

* More arrivals and shitty uhh preload

* cvar

* a

* clockin + maps

* shitter prevention

* Placement

* a

* cvar for tests and dev

* weh

Content.Shared/Tiles/FloorTileAttemptEvent.cs

@@ -0,0 +1,7 @@
+namespace Content.Shared.Tiles;
+
+/// <summary>
+/// Raised directed on a grid when attempting a floor tile placement.
+/// </summary>
+[ByRefEvent]
+public record struct FloorTileAttemptEvent(bool Cancelled);