interruptTurn seals bufRef at the keypress and drops later message.delta
frames, but the agent keeps streaming until it notices the interrupt and
persists every delta it delivered. Deltas already in flight were saved to
state.db (and replayed next turn) without ever being drawn.
The interrupted message.complete carries that persisted partial; when it
extends the sealed text, the interrupted bubble now takes it (or a bubble
is added when nothing had been shown yet).
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