everyByteValueSurvivesTheRoundTrip

Values are opaque binary, so all 256 byte values must survive — the high half especially.

The knob here is the byte range, and narrowing it switches this property off entirely: a payload of 1, 2, 3 is what the pre-existing per-backend tests mostly used, and every byte in it is below 0x80. A backend crossing a boundary that widens a byte — a JS Uint8Array read back without sign extension, a text encoding applied to what is not text, a Char-based conversion — round-trips 1, 2, 3 perfectly and mangles everything from 0x80 up.