Add AnimusSystem, a client side state machine focused on animations and modularity.

Robust.Client/Animus/Actions/AnimusActionAnimationBase.cs

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+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Numerics;
+using JetBrains.Annotations;
+using Robust.Client.Animations;
+using Robust.Client.GameObjects;
+using Robust.Shared.Animations;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Maths;
+using Robust.Shared.Serialization.Manager.Attributes;
+
+namespace Robust.Client.Animus.Actions;
+
+[ImplicitDataDefinitionForInheritors]
+[PublicAPI]
+public abstract partial class AnimusActionAnimationBase
+{
+    /// <summary>
+    /// Simple stopping animation for cases where only rotation and offset were affected.
+    /// </summary>
+    protected static readonly Animation StopAnimation = new()
+    {
+        Length = TimeSpan.FromSeconds(0),
+        AnimationTracks =
+        {
+            new AnimationTrackComponentProperty()
+            {
+                ComponentType = typeof(SpriteComponent),
+                Property = nameof(SpriteComponent.Rotation),
+                InterpolationMode = AnimationInterpolationMode.Linear,
+                KeyFrames =
+                {
+                    new AnimationTrackProperty.KeyFrame(Angle.FromDegrees(0), 0),
+                },
+            },
+            new AnimationTrackComponentProperty()
+            {
+                ComponentType = typeof(SpriteComponent),
+                Property = nameof(SpriteComponent.Offset),
+                InterpolationMode = AnimationInterpolationMode.Linear,
+                KeyFrames =
+                {
+                    new AnimationTrackProperty.KeyFrame(new Vector2(), 0),
+                },
+            },
+        },
+    };
+
+    public virtual void Initialize(EntityManager entityManager)
+    {
+
+    }
+
+    protected abstract Animation? GetNextAnimation(AppearanceSystem appearanceSystem, EntityUid entity, bool restarting);
+
+    protected abstract Animation? GetStopAnimation(AppearanceSystem appearanceSystem, EntityUid entity);
+
+    internal bool TryNextAnimation(AppearanceSystem appearanceSystem, EntityUid entity, [NotNullWhen(true)] out Animation? anim, bool restarting)
+    {
+        anim = GetNextAnimation(appearanceSystem, entity, restarting);
+        return anim != null;
+    }
+
+    internal bool TryStopAnimation(AppearanceSystem appearanceSystem, EntityUid entity, [NotNullWhen(true)] out Animation? anim)
+    {
+        anim = GetStopAnimation(appearanceSystem, entity);
+        return anim != null;
+    }
+}
+
+public sealed partial class AnimusActionAnimationNull : AnimusActionAnimationBase
+{
+    protected override Animation? GetNextAnimation(AppearanceSystem appearanceSystem, EntityUid entity, bool restarting)
+    {
+        return null;
+    }
+
+    protected override Animation? GetStopAnimation(AppearanceSystem appearanceSystem, EntityUid entity)
+    {
+        return null;
+    }
+}

Robust.Client/Animus/Actions/AnimusActionSpriteChangeBase.cs

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+using JetBrains.Annotations;
+using Robust.Client.GameObjects;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization.Manager.Attributes;
+
+namespace Robust.Client.Animus.Actions;
+
+[ImplicitDataDefinitionForInheritors]
+[PublicAPI]
+public abstract partial class AnimusActionSpriteChangeBase
+{
+    public abstract void Initialize(Entity<SpriteComponent> entity, EntityManager entityManager);
+    public abstract void ExecuteSpriteChange(Entity<SpriteComponent> entity);
+    public abstract void ResetSpriteChange(Entity<SpriteComponent> entity);
+}
+
+public sealed partial class AnimusActionSpriteChangeNull : AnimusActionSpriteChangeBase
+{
+    public override void Initialize(Entity<SpriteComponent> entity, EntityManager entityManager)
+    {
+
+    }
+
+    public override void ExecuteSpriteChange(Entity<SpriteComponent> entity)
+    {
+
+    }
+
+    public override void ResetSpriteChange(Entity<SpriteComponent> entity)
+    {
+
+    }
+}

Robust.Client/Animus/AnimusComponent.cs

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+using System;
+using System.Collections.Generic;
+using Robust.Client.Animus.States;
+using Robust.Client.Animus.Timers;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization.Manager.Attributes;
+
+namespace Robust.Client.Animus;
+
+[RegisterComponent]
+public sealed partial class AnimusComponent : Component
+{
+    [DataField]
+    internal List<ProtoId<AnimusPrototype>> StateMachines;
+
+    internal readonly List<AnimusInstance> ActiveStateMachines = [];
+}
+
+/// <summary>
+/// Prototypes are global instances but conditions and other objects may require per-instance data.
+/// </summary>
+internal sealed class AnimusInstance
+{
+    internal ProtoId<AnimusPrototype> Prototype = default;
+    internal AnimusStateBase[] States = [];
+    internal AnimusTimerBase? Timer = null;
+    internal AnimusStateBase ActiveState = new AnimusStateNull();
+    internal AnimusStateBase DefaultState = new AnimusStateNull();
+    internal TimeSpan ActiveStateExitTime = TimeSpan.Zero;
+}

Robust.Client/Animus/AnimusPrototype.cs

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+using System.Collections.Generic;
+using Robust.Client.Animus.States;
+using Robust.Client.Animus.Timers;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization.Manager.Attributes;
+using Robust.Shared.ViewVariables;
+
+namespace Robust.Client.Animus;
+
+[Prototype]
+internal sealed partial class AnimusPrototype : IPrototype
+{
+    [ViewVariables]
+    [IdDataField]
+    public string ID { get; private set; } = default!;
+
+    /// <summary>
+    /// A collection of possible states for this animus.
+    /// </summary>
+    [DataField]
+    internal List<AnimusStateBase> States = [];
+
+    /// <summary>
+    /// Optional timer for executing an update.
+    /// Setting this disables periodic condition checks.
+    /// </summary>
+    [DataField]
+    internal AnimusTimerBase? Timer;
+
+    /// <summary>
+    /// The default state to enter when no other state matches their conditions.
+    /// </summary>
+    [DataField]
+    internal AnimusStateBase DefaultState = new AnimusStateNull();
+
+}