WINDOWS-1252 UNDEFINED-BYTE ACCEPTANCE
The byte-exact golden Receipt is bound to inputs/undefined_81.csv = 41 81 42.
Expected first undefined byte: 0x81 at zero-based absolute physical offset 1.
Transcoding does not start; source is unchanged; OUTPUT_FILENAME is NONE.

Additional inputs cover every frozen undefined Windows-1252 byte and a first-vs-later case.
Buffer/window-size invariance is an execution-seam acceptance obligation; the expected absolute offset does not change.
