Ambient music rules refactor (#29921)

* refactor

* dutypo

Content.Shared/Random/Rules/OnMapGrid.cs

@@ -0,0 +1,19 @@
+namespace Content.Shared.Random.Rules;
+
+/// <summary>
+/// Returns true if griduid and mapuid match (AKA on 'planet').
+/// </summary>
+public sealed partial class OnMapGridRule : RulesRule
+{
+    public override bool Check(EntityManager entManager, EntityUid uid)
+    {
+        if (!entManager.TryGetComponent(uid, out TransformComponent? xform) ||
+            xform.GridUid != xform.MapUid ||
+            xform.MapUid == null)
+        {
+            return Inverted;
+        }
+
+        return !Inverted;
+    }
+}