The pre-join check charged the whole unframed buffer, which legitimately holds
several frames each within FRAME_PARSE_CAP_BYTES: a first frame of exactly the
cap followed by a second frame crossed the counter and was misreported as a
worker-exit. The pre-join rejection now fires only while the held bytes are a
single unframed line (this chunk carries no newline); once a newline arrives,
a FIRST-FRAME check measures the bytes up to the first newline across the held
chunks (including sealed blocks) and rejects only that frame before the join —
keeping the peak at one copy of its wire bytes — while later frames in the
same buffer are handled by the restored per-line check. Regression cases: a
72 MiB newline-free buffer is rejected pre-join (fail-before: joining would
have doubled it); two within-cap frames whose combined buffer crosses the cap
both survive (fail-before: the unconditional counter check turns it red).
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