ChatView's bottom-follow recognized only wheel gestures as reader input,
so touch panning, scrollbar dragging, and keyboard paging could not leave
the bottom of a streaming transcript. Replace the wheel listener with
device-agnostic attribution: a scroll position deviating from the
observed-top ledger of the last delivered or written scrollTop is reader
input. Adds keyboard-paging and touch-style fling e2e scenarios (red under
the old implementation) and the bilingual Agent Note triplet.
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