The tail-copy regression built `"first\n" + "A" * 200 MiB`, whose construction
alone peaks near 400 MiB (the string plus the concat temporary) and OOMs under
the 384 MiB addressSpaceMb before the log path under test runs — a MemoryError in
the model, not the defect. Build the tail in a variable and concatenate only the
newline (peak ~2x150 MiB = 300 MiB, under the address space), so the model's own
allocation fits; the pre-fix code then buffered the whole 150 MiB tail again,
pushing past 384 MiB, while the sliced prefix does not.
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