completeAssistantMessage merged a turn's final text onto its sealed interim
bubble only when the session's volatile interimBoundaryPending flag was still
true. A subsequent message.start (chained turn, follow-up, or mid-turn
compaction) resets that flag to false; when it landed between the same turn's
message.interim and message.complete, the flag-based gate fell through and the
UI appended a duplicate bubble.
Key the merge on the message's OWN durable interim state instead of the
session flag. finalContinuesInterim already requires existing.interim plus
prefix continuity, so distinct replies (which don't continue the interim) are
still appended as their own bubble.
Adds a regression test: interim + message.start + completing final that
continues the interim must yield one bubble, not two.
spfcraze
71a98f69ee