Extends the live-message guard to reject both _live assistant rows and
_pending user rows that haven't been committed server-side yet. Adds a
belt-and-suspenders check: when S.busy is true, the last message in
S.messages is also blocked regardless of flags, closing a timing window
where _live might be momentarily absent on the streaming row.
The user's optimistically-pushed message (messages.js:1603) now carries
_pending:true so forkFromMessage can distinguish it from committed
messages.
kertisalex
90ee9484e0
