Streamdown runs our `preprocess` inside its own useMemo, and the user
bubble runs `extractEmbeddedImages`/directive parsing inside theirs — so
anything thrown while rendering one message (a regex/stack overflow on
adversarial content) escapes to the ROOT error boundary and takes down
the entire app, as seen in a reported `RangeError: Maximum call stack
size exceeded` from a single message.
Wrap both the assistant preprocess pipeline and the user-message
directive passes in try/catch that degrade to the raw text. One bad
message now renders plain instead of nuking the transcript.
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