test(queue): cover multi-message continuation collapse

Add a frontend store test (flagged by a Gemini review) that queues TWO
messages mid-turn and asserts they collapse into a single untagged
initiator row joined with "\n---\n" — matching the backend's joined user
turn — and that the next turn-start tags that single row. The prior test
only covered the single-message case, leaving the join logic structurally
correct but untested.

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