Document the Native Animated architecture (#58106)

Summary:

Document the cross-platform C++ implementation of Native Animated, including its purpose, execution model, core components, prop application paths, and relationship with the shared Animation Backend.

Changelog:
[Internal]

___

Differential Revision: D116797721

Author: zeyap

__docs__/README.md

@@ -50,6 +50,7 @@ TODO: Explain the different components of React Native at a high level.
       - Layout
       - Mounting
       - [Animation Backend](../packages/react-native/ReactCommon/react/renderer/animationbackend/__docs__/AnimationBackend.md)
+      - [Native Animated](../packages/react-native/ReactCommon/react/renderer/animated/__docs__/NativeAnimated.md)
     - Native Modules / TurboModules
     - JS Runtime
       - [Event Loop](../packages/react-native/ReactCommon/react/renderer/runtimescheduler/__docs__/README.md)

packages/react-native/ReactCommon/react/renderer/animated/__docs__/NativeAnimated.md

@@ -0,0 +1,100 @@
+# Native Animated
+
+[🏠 Home](../../../../../../../__docs__/README.md)
+
+Native Animated is the cross-platform C++ native driver for React Native's
+`Animated` API. The JavaScript library sends a graph of values, operations,
+styles, and transforms through the `NativeAnimatedModule` TurboModule. Native
+Animated evaluates that graph off the JavaScript thread and applies the
+resulting props to views.
+
+The main purpose of this C++ implementation is to let platforms share the graph
+and driver logic instead of maintaining separate native implementations. It
+integrates Native Animated with the shared
+[Animation Backend](../../animationbackend/__docs__/AnimationBackend.md), which
+updates layout and non-layout props through Fabric and keeps those updates
+synchronized with React commits.
+
+## 🚀 Usage
+
+Application code uses the public JavaScript `Animated` API rather than this
+package directly. A typical animation:
+
+1. Creates and connects animated nodes.
+2. Starts a time-based animation or attaches an event mapping.
+3. Evaluates affected nodes each frame, applies props, and invokes the
+   completion callback.
+
+JavaScript batches graph operations with `startOperationBatch` and
+`finishOperationBatch` so they are applied together on the native side.
+`queueAndExecuteBatchedOperations` is specific to the legacy Android
+implementation and is not implemented by this C++ module.
+
+## 📐 Design
+
+Native Animated maintains a directed acyclic graph. Animation drivers and native
+events update value nodes, changes propagate through dependent nodes, and
+`PropsAnimatedNode` collects the props to apply to a view. Dirty tracking limits
+each frame to nodes affected by changed values.
+
+### Core components
+
+- `AnimatedModule` (`AnimatedModule.h`) implements the `NativeAnimatedModule`
+  spec. It buffers graph operations from JavaScript and schedules them on the
+  render thread.
+- `NativeAnimatedNodesManager` (`NativeAnimatedNodesManager.h`) owns the graph,
+  animation drivers, and event drivers. It evaluates updates, schedules prop
+  commits, and invokes listeners and completion callbacks.
+- `NativeAnimatedNodesManagerProvider` (`NativeAnimatedNodesManagerProvider.h`)
+  creates one shared manager per runtime, connects it to `UIManager`, selects
+  the prop-application path, and registers native event delivery.
+
+### Animated nodes
+
+All nodes derive from `AnimatedNode` (`nodes/AnimatedNode.h`). The main groups
+are:
+
+- `ValueAnimatedNode`, which stores the scalar values written by animations and
+  events.
+- Operator, interpolation, and color nodes, which derive values from other
+  nodes.
+- `StyleAnimatedNode`, `TransformAnimatedNode`, and `ObjectAnimatedNode`, which
+  assemble structured output.
+- `TrackingAnimatedNode`, which follows another value through an animation.
+- `PropsAnimatedNode`, which collects output for a view or `ShadowNodeFamily`.
+
+### Animation and event drivers
+
+Drivers derived from `AnimationDriver` (`drivers/AnimationDriver.h`) update a
+single `ValueAnimatedNode` on each frame. Native Animated supports frame-based
+timing, spring, and decay drivers.
+
+`EventAnimationDriver` (`event_drivers/EventAnimationDriver.h`) maps a path in a
+native event payload, such as `contentOffset.y`, to a value node. This lets
+events drive the graph without a JavaScript round trip.
+
+### Applying props
+
+Native Animated supports two prop-application paths:
+
+- The default path directly updates non-layout props and uses a Fabric commit
+  for layout props. `MergedValueDispatcher` coalesces updates, while
+  `AnimatedMountingOverrideDelegate` prevents React commits from overwriting
+  animated values.
+- The shared Animation Backend path delegates per-frame mutations to the
+  [Animation Backend](../../animationbackend/__docs__/AnimationBackend.md),
+  which applies them and coordinates with React commits.
+
+### Threading
+
+The JavaScript thread only buffers TurboModule operations. Graph mutation,
+driver updates, evaluation, and prop commits run on the render thread using the
+platform frame callback. The manager uses locks for state shared between the two
+threads.
+
+## 🔗 Relationships
+
+- The JavaScript `Animated` library drives Native Animated through the
+  `NativeAnimatedModule` TurboModule.
+- Native Animated can delegate prop application to the shared
+  [Animation Backend](../../animationbackend/__docs__/AnimationBackend.md).