Adds event for mass and angular inertia changes. (#5286)

* Adds MassDataChangedEvent to physics
This event is raised in response to changes to an entities innate mass/angular inertia/center of mass

* Use properties to fetch data from component

* Comp1

* Vector2

* I sure love an analyzer that doesn't work half the time

Robust.Shared/Physics/Events/MassChangedEvent.cs

@@ -0,0 +1,28 @@
+using Robust.Shared.GameObjects;
+using Robust.Shared.Physics.Components;
+using System.Numerics;
+
+namespace Robust.Shared.Physics.Events;
+
+/// <summary>
+/// By-ref directed event raised when the mass or angular inertia or center of mass of a physics body changes.
+/// </summary>
+/// <param name="Entity">The physics body that changed.</param>
+/// <param name="OldMass">The old mass of the physics body.</param>
+/// <param name="OldInertia">The old angular inertia of the physics body.</param>
+/// <param name="OldCenter">The old (local) center of mass of the physics body.</param>
+[ByRefEvent]
+public readonly record struct MassDataChangedEvent(
+    Entity<PhysicsComponent, FixturesComponent> Entity,
+    float OldMass,
+    float OldInertia,
+    Vector2 OldCenter
+)
+{
+    public float NewMass => Entity.Comp1._mass;
+    public float NewInertia => Entity.Comp1._inertia;
+    public Vector2 NewCenter => Entity.Comp1._localCenter;
+    public bool MassChanged => NewMass != OldMass;
+    public bool InertiaChanged => NewInertia != OldInertia;
+    public bool CenterChanged => NewCenter != OldCenter;
+}

Robust.Shared/Physics/Systems/SharedPhysicsSystem.Components.cs

@@ -256,6 +256,9 @@ public void ResetMassData(EntityUid uid, FixturesComponent? manager = null, Phys
         if (!_fixturesQuery.Resolve(uid, ref manager))
             return;
 
+        var oldMass = body._mass;
+        var oldInertia = body._inertia;
+
         body._mass = 0.0f;
         body._invMass = 0.0f;
         body._inertia = 0.0f;
@@ -314,6 +317,12 @@ public void ResetMassData(EntityUid uid, FixturesComponent? manager = null, Phys
         // Update center of mass velocity.
         body.LinearVelocity += Vector2Helpers.Cross(body.AngularVelocity, localCenter - oldCenter);
         Dirty(uid, body);
+
+        if (body._mass == oldMass && body._inertia == oldInertia && oldCenter == localCenter)
+            return;
+
+        var ev = new MassDataChangedEvent((uid, body, manager), oldMass, oldInertia, oldCenter);
+        RaiseLocalEvent(uid, ref ev);
     }
 
     public bool SetAngularVelocity(EntityUid uid, float value, bool dirty = true, FixturesComponent? manager = null, PhysicsComponent? body = null)