chore(changeset): note that @cipherstash/auth ships stack-auth 0.43.0

The native binding embeds the stack-auth crate version in its user
agent, so bumping stack-auth to 0.43.0 changes what @cipherstash/auth
ships. The require-auth-npm-changeset check rightly fails without this
changeset.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PaY5xYydZUWhv8Nex9Sw8a

Author: auxesis

.changeset/auth-stack-auth-0-43-0.md

@@ -0,0 +1,7 @@
+---
+"@cipherstash/auth": patch
+---
+
+The native binding is now built from the `stack-auth` 0.43.0 crate, so its
+requests identify themselves as `stack-auth/0.43.0` in the user agent. No API
+or behaviour change for `@cipherstash/auth` consumers.