docs(changelog): note the DynamoDB concurrency fix under [Unreleased]

Flags the behaviour change for callers whose previously-racing
UpdateItem/PutItem/TransactWriteItems calls will now see the
ConditionalCheckFailedException or TransactionCanceledException that real
DynamoDB would raise.

Author: stashev

CHANGELOG.md

@@ -14,6 +14,7 @@
 * include `ec2` and `ecs` in enabled-service reporting and enforce disabled gating for ACM and ECS targeted requests
 * return protocol-correct JSON disabled responses for auth-only REST GETs instead of falling back to XML
 * honor `floci.storage.services.acm.*` overrides in `StorageFactory`
+* **dynamodb:** serialise concurrent mutations via per-item locks and total-ordered transaction lock acquisition so `UpdateItem` counters, conditional `PutItem`/`DeleteItem`, and `TransactWriteItems` are linearisable under load — callers whose prior races silently succeeded may now see the `ConditionalCheckFailedException` / `TransactionCanceledException` that real DynamoDB would raise ([#571](https://github.com/floci-io/floci/issues/571))
 * **s3:** honor canned object ACLs on PutObject, CopyObject, and multipart uploads
 
 ## [1.5.2](https://github.com/floci-io/floci/compare/1.5.1...1.5.2) (2026-04-10)