Skip redundant processing when coalesced chunks result in identical
content. During rapid streaming, multiple chunks may arrive and coalesce
into pendingMarkdown while processing is ongoing. When the final
coalesced content equals what was just processed, we can skip entirely.
Also clarify the RAF yield comment: the key insight is that chunks
arriving during the yield naturally coalesce, so we always render
the latest state without explicitly tracking what to skip.
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