assign_message

Attach a transcript message and refresh idx display when indices arrive asynchronously.

Replaces any prior watcher on the previous message. If

 idx_first 

and

 idx_last 

are already set, notifies

 msg_idx_txt 

once; otherwise connects to

 idx-last 

on the message until both indices are known.

Parameters:

m	message to bind (content-stream or similar)