A caption-less image echo keeps its captured tail because it counts image turns
after it — but a send issued before any history was known captured null, which
counts from the top of the transcript. An old image turn then claimed the send
and bound the user's fresh photo to it, leaving the just-sent turn with no
preview. A null tail is not a boundary worth preserving, so pin those too.
`transcriptSettled` was optional and defaulted to the gate it replaced, so any
caller that omitted it silently got the pre-fix behaviour. Required now, and
threaded through every harness.
Brennan Benson
4db02de8a0