docs(changelog): add go-ds-flatfs atomic batch writes

documents the new flatfs batch implementation that uses atomic
operations via temp directory, preventing orphan blocks on interrupted
imports and reducing memory usage.

Author: lidel

docs/changelogs/v0.40.md

@@ -10,15 +10,26 @@ This release was brought to you by the [Shipyard](https://ipshipyard.com/) team.
 
 - [Overview](#overview)
 - [🔦 Highlights](#-highlights)
+  - [Atomic batch writes in flatfs datastore](#atomic-batch-writes-in-flatfs-datastore)
   - [Routing V1 HTTP API now exposed by default](#routing-v1-http-api-now-exposed-by-default)
-  - [Track total size when adding pins](#track-total-size-when-adding-pins]
+  - [Track total size when adding pins](#track-total-size-when-adding-pins)
 - [📝 Changelog](#-changelog)
 - [👨‍👩‍👧‍👦 Contributors](#-contributors)
 
 ### Overview
 
 ### 🔦 Highlights
 
+#### Atomic batch writes in flatfs datastore
+
+The flatfs datastore batch implementation has been rewritten to use atomic operations. Blocks are now written to a temporary directory first and renamed to their final locations only on commit.
+
+If kubo is interrupted during `ipfs add` or `ipfs dag import`, uncommitted blocks are automatically cleaned up on the next start, preventing orphan blocks from polluting the datastore. Previously, interrupted imports could leave partial data behind that was difficult to identify and clean up.
+
+This also reduces memory usage during batch operations, as blocks are written to disk asynchronously instead of being buffered in RAM.
+
+See [go-ds-flatfs#142](https://github.com/ipfs/go-ds-flatfs/pull/142) for implementation details.
+
 #### Routing V1 HTTP API now exposed by default
 
 The [Routing V1 HTTP API](https://specs.ipfs.tech/routing/http-routing-v1/) is now exposed by default at `http://127.0.0.1:8080/routing/v1`. This allows light clients in browsers to use Kubo Gateway as a delegated routing backend instead of running a full DHT client. Support for [IPIP-476: Delegated Routing DHT Closest Peers API](https://github.com/ipfs/specs/pull/476) is included. Can be disabled via [`Gateway.ExposeRoutingAPI`](https://github.com/ipfs/kubo/blob/master/docs/config.md#gatewayexposeroutingapi).