feat(keyring-sdk): add AtomicKeyring and ReadWriteLock helper

packages/keyring-sdk/CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Add `AtomicKeyring` base class, `AtomicUpdater` type, and `isAtomicKeyring` helper for keyrings that serialize their own operations and commit state through controller-mediated vault writes ([#640](https://github.com/MetaMask/accounts/pull/640))
+  - Atomic keyrings run long-running operations inside their own read-write lock, entirely outside the controller mutex, and persist state through the injected `update` callback.
+- Add `ReadWriteLock` primitive with parallel readers, exclusive writers ([#640](https://github.com/MetaMask/accounts/pull/640))
+  - The lock is write-preferring by default: a pending writer blocks new readers, so a steady stream of reads cannot starve a long write.
+  - Pass `priority: 'read'` to let new readers join active readers even while a writer is queued, at the cost that continuous reads can starve a queued writer.
+
 ### Changed
 
 - **BREAKING:** Remove envelope from migration framework ([#619](https://github.com/MetaMask/accounts/pull/619))