blog: illustrate stack deformation across ticks

Update featured image to show async deforming the call stack: a then() callback runs on the next tick with restored tags, so group attribution changes. Add a short bridge paragraph tying the diagram to k6 tag behavior.

Session-ID: ses_39d99b9c2ffeSKxHPZAk9P7E1O

Author: taras

www/blog/2026-02-15-k6-structured-concurrency/index.md

@@ -15,6 +15,11 @@ expect to work with sync `group()` doesn't work once async gets introduced.
 This post is about using structured concurrency to align k6's JavaScript runtime
 with your expectations.
 
+The diagram at the top shows what goes wrong. Async doesn't just add time, it
+deforms your call stack. Some code runs on the stack you are in now, and some
+code runs on a new stack on the next tick. When that happens, the "current" tags
+are different, because `group()` has already unwound and restored them.
+
 ## Why `group()` can't fix this on its own
 
 k6's `group()` behaves like a `try/finally`-scoped tag mutation: set a tag,