Fix a bug in continuation capture across multiple stack chunks

This patch fixes a bug in continuation capture when the prompt is not in the current stack chunk.

The original code was simply wrong, and I’d even somewhat suspected as much, so I really ought to have caught this in the original MR. Unfortunately, the test cases I’d previously tried writing weren’t quite enough to tickle the bug. Fortunately, that is now corrected: this patch includes both a fix and a regression test, so I now feel pretty confident about the correctness of the implementation.