feat(graphics): improve rendering pipeline and docs

- Refactor D3D12 backend and RenderGraph module
- Update graphics RHI and core rendering components
- Add Random.hlsl shader include
- Regenerate API documentation and update user guides

doc/docs/systems.md

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+# Systems Overview
+
+Systems are where all your ECS logic lives. They interact with data by iterating over entities that match an `EntityQuery`.
+
+## The `ISystem` Interface
+
+At its core, a system implements `ISystem`, which provides three lifecycle methods:
+
+```csharp
+public interface ISystem
+{
+    void Initialize(ref readonly SystemAPI systemAPI);
+    void Update(ref readonly SystemAPI systemAPI);
+    void Cleanup(ref readonly SystemAPI systemAPI);
+}
+```
+
+The `SystemAPI` struct provides access to the `World` and `TimeData` for the current frame.
+
+## `SystemBase`
+
+For most gameplay code, you should inherit from the abstract class `SystemBase`, which provides convenient wrappers and handles "Should Update" logic automatically.
+
+```csharp
+public class MovementSystem : SystemBase
+{
+    private Identifier<EntityQuery> _query;
+
+    protected override void OnInitialize(ref readonly SystemAPI systemAPI)
+    {
+        // Build a query for entities that have both Position and Velocity
+        _query = new QueryBuilder()
+            .WithAllRW<Position>()
+            .WithAll<Velocity>()
+            .Build(World);
+
+        // Tell the system base to ONLY run OnUpdate if this query has at least 1 entity
+        RequireQueryForUpdate(_query);
+    }
+
+    protected override void OnUpdate(ref readonly SystemAPI systemAPI)
+    {
+        float dt = systemAPI.Time.DeltaTime;
+        
+        ref var query = ref World.ComponentManager.GetEntityQueryReference(_query);
+        
+        foreach (var chunk in query.GetChunkIterator())
+        {
+            var positions = chunk.GetComponentDataRW<Position>();
+            var velocities = chunk.GetComponentData<Velocity>();
+
+            for (int i = 0; i < chunk.EntityCount; i++)
+            {
+                positions[i].Value += velocities[i].Value * dt;
+            }
+        }
+    }
+}
+```
+
+### Automatic Update Triggers
+
+Notice the `RequireQueryForUpdate(_query)` call. `SystemBase` will skip calling `OnUpdate` if the query results are completely empty, saving processing time. `SystemBase` also invokes `OnStartRunning` and `OnStopRunning` when the system transitions between having matching entities and not.
+
+## System Execution Order
+
+By default, systems execute in the order they are added to a `SystemGroup`. To enforce execution order regardless of initialization sequence, use the `[UpdateAfter]` and `[UpdateBefore]` attributes.
+
+```csharp
+[UpdateAfter(typeof(PhysicsSystem))]
+[UpdateBefore(typeof(RenderSystem))]
+public class MovementSystem : SystemBase
+{
+    // ...
+}
+```
+
+When you call `group.SortSystems()`, the `SystemGroup` uses Kahn's algorithm to perform a topological sort and resolves these dependencies. If circular dependencies are detected, the engine will throw an exception.
+
+## System Groups
+
+`SystemGroup`s implement `ISystem` themselves, meaning you can nest groups within groups to build complex hierarchical update loops (e.g. `FixedUpdateGroup`, `PresentationGroup`). GhostEngine provides a `DefaultSystemGroup` that is automatically instantiated by the `SystemManager`.