fix(sdk): document flush() first-error rejection semantics

Promise.all rejects on first write failure; errors also surface via
persist:error event channel per write.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Co-authored-by: Miguel Ángel <miguel07alm@protonmail.com>

Author: vanceingalls

packages/sdk/src/adapters/fs.ts

@@ -55,6 +55,8 @@ class FsAdapter implements PersistAdapter {
   }
 
   async flush(): Promise<void> {
+    // Promise.all rejects on the first write failure; per-write errors are also
+    // surfaced individually through the persist:error event channel.
     await Promise.all([...this.inflightWrites]);
   }