The decoder recovered offset-encoded 16-bit string lengths into a
uint16_t, so sz += ProtocolOffset8Bit truncated lengths in [65536,
65791] back into [0, 255] in release builds, desynchronizing the
stream. Read the wire value into uint16_t sz16 and recover into a
uint32_t sz, relying on automatic promotion for the addition.
Align the client asserts with the encoder's actual capacity
(ProtocolOffset8Bit + uint16 max).
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