ReadAsync allocated a fresh 4-byte array per inbound frame; the GWC-08
pass pooled the payload buffer but left the prefix. Replaced with a
per-instance scratch field — the reader is single-consumer by
construction (one read loop per WorkerClient, handshake reads complete
before the loop starts), so a per-instance buffer is safe and the
non-reentrancy that makes it safe is now stated on the class. Pooling
four bytes via ArrayPool would cost more than the allocation it saves.
Tests: WorkerFrameProtocolTests.ReadAsync_WithMultipleFramesOnOneReader_
ParsesEveryFrame reads five frames of differing payload length through
one reader, so a stale prefix carried between calls would misparse.
Joseph Doherty
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